KSeF number on the invoice. What is an e-invoice identifier and where to find it in the system

If you come here with the question “where is the KSeF number on the invoice”, that's good. In KSeF, the topic looks different than in the world of PDFs and emails. In the National E-invoice System, an e-invoice is a document in the system. and the KSeF number of the invoice is a unique number that is used to uniquely identify the invoice in circulation.

In a nutshell. The invoice number assigned by the taxpayer and the KSeF number are two different designations. KSeF does not replace the invoice number. Your numbering continues and the KSeF number is automatically assigned by the KSeF system after sending and accepting the e-invoice. Therefore, the KSeF number is visible in specific places in the system and in the official acceptance certificate. You will not always see it in the body of the invoice that you have as a printout or PDF.

In this article I show you:
— what is the KSeF number and why is it a unique string that identifies the invoice in the system
— what the structure of the KSeF number looks like and why it consists of 35 characters
— where to check the number when you have invoices issued but you don't see the tag on the invoice preview
— what does the absence of a KSeF number mean and how it affects access to the invoice and the status of acceptance of the document
— how to cover the payment and transfer, including the KSeF number in the transfer title, also with the split payment mechanism and invoice payments

If you have the implementation of KSeF on your radar, especially in the context of February 1, 2026 and KSeF 2.0, this guide will help you go from “where it is” to “I have it, copied, it works” without wandering around the system.

National e-invoicing system: how it works and what VAT means

The national e-invoice system is a central KSeF system, in which the e-invoice ceases to be just a file to be sent, and becomes a document in the system. This is important because it affects how VAT works, how invoicing looks and when you can be sure that invoices in the system have actually been accepted.

In practice, it looks like this:

— The invoice is prepared and sent to the KSeF system. You can do this in the application, through the integrated program or through the KSeF API
— The KSeF system checks the correctness of the data and accepts the document
— After accepting the invoice, the system assigns it a KSeF number. This is a unique number that is automatically assigned by the system. This number is generated only when the invoice is accepted

Taxpayer and document: how the KSeF e-invoice circulation works

In the circulation of KSEF, three things are crucial: the taxpayer, the document and the system.

— The taxpayer sends an e-invoice to the system
— The document goes into circulation as one of the structured invoices
— The KSeF system assigns an identifier that allows unambiguous identification of the invoice in the system and identification of invoices in circulation

It is worth separating it, because the concepts are often confused:

— The invoice number assigned by the taxpayer is an element of numbering. This is your own invoice number, usually as a sequential number assigned within one series or within one or more series
— The KSeF number of the invoice is something else. This is the number that identifies the invoice in KSeF, which is a unique string of characters assigned automatically by the system. This number identifies the invoice in the KSeF system

Therefore, KSeF does not replace the invoice number assigned in your numbering. In practice, you have two numbers that work in parallel and each has a different task.

E-invoice in practice: when a structured invoice is recognized

The simplest rule is this: a structured invoice is recognized when the system has accepted it. Just generating a document in the program is not yet the same as accepting an invoice in KSeF.

After accepting the invoice:

— the KSeF number is assigned automatically by the system
— the KSeF number is visible in the system and in the official acceptance certificate
— thanks to this you have access to the invoice and confirmation that the document is in the system

This is why the KSeF number is a unique identifier. It is assigned to a specific e-invoice and allows a certain identification of the document, even if a PDF or print circulates in circulation.

Invoices in KSeF arrive: why KSeF becomes an important reference point

With the implementation of KSeF in the phase-mandatory model, invoices issued in this standard will become more and more frequent. This makes the topic of the KSeF number come back every day, because it applies to every invoice and often also payment for invoices, when you need to quickly find the right number or confirm that the document is in the system.

It is worth remembering one more thing. The KSeF number is assigned to the invoice in the system, but whether you will see it “on the invoice” in the sense of a PDF preview or a printout depends on how the visualization works in a particular tool. In many cases, you will probably check the KSeF number in the system and in the confirmation of acceptance of the document.

Invoice KSeF number: Invoice identification number in KSeF and number structure

The KSeF number of the invoice is the number identifying the invoice in the KSeF, i.e. a unique number that is automatically assigned by the KSeF system after acceptance of the document. In practice, it acts as an identifier that uniquely identifies the invoice in the National E-Invoice System, regardless of what invoice number appears in your numbering.

It is worth remembering one thing. The KSeF number is a unique character string assigned to a specific document. This is not a “sequential” numbering as in an invoice number.

KSeF consists of 35 characters: how to read the number

The structure of the KSeF number has a fixed form. It consists of 35 characters and is unique for each e-invoice accepted in the system.

What does this mean in practice:

— The KSeF number is not a “nice number” that is easy to remember. It is a unique character string that is automatically assigned by the system
— the number assigned by the KSeF is fixed for a given invoice. Doesn't change when you download, print, or forward
— the number assigned in the KSeF is used to identify the invoice in the system, especially when you need to quickly find a document or confirm its status

If someone asks for a “number”, clarify whether it is a KSeF number or an invoice number. In the KSeF system, both can be seen, but they perform different functions.

Invoice number issued by the taxpayer: numbering and invoice number

The invoice number assigned by the taxpayer is your internal numbering, that is, the invoice number resulting from the adopted scheme. This can be another number assigned within one series, or within one or more series, according to the rules you use when invoicing.

The most important distinction:

— the invoice number given by the taxpayer is used for order in your documents and processes
— KSeF number is a unique number assigned automatically by the system that identifies the invoice in the KSeF system
— KSeF does not replace the invoice number issued by the taxpayer. These two numbers work in parallel

Therefore, on one invoice you can have both the invoice number and the invoice KSeF number at the same time. One is part of the numbering and the other is an identifier in the National E-Invoice System.

KSeF number: identifier and KSeF number. Where do you see it and how to copy

The KSeF number is visible in the places that confirm the acceptance of the invoice in the system. Most often you will see it where the KSeF system shows the details of the document.

Where to check the number:

— in the KSeF system, in the details of the document, after entering the list of invoices in the KSeF system and opening a specific item
— in the KSeF taxpayer's application, in the invoice view, where the KSeF number is visible as the field “KSeF number” or “KSeF number”
— in the official acknowledgement of receipt, as confirmation of acceptance of the e-invoice
— in a program integrated with the KSeF API, usually in the “KSeF number” field pinned to the document

How to copy a number without errors:

— open the document in the KSeF system or in the KSeF taxpayer application
— find the field with the invoice KSeF number
— use the copy function instead of rewriting manually, because the unique string is easy to confuse
— if the number is not available, go back to the invoice acceptance information. The absence of a KSeF number usually means that the document has not yet been accepted or is not yet visible in circulation

Bulk ID: when the KSeF collective ID appears

The Bulk ID appears when a shipment containing more than one document enters the KSeF system, or when the utility sends invoices in “packages” and needs a single number to track the entire process. In practice, the KSeF Aggregate Identifier helps determine what happened to the shipment before each invoice gets its own KSeF invoice number upon receipt.

The most important distinction is simple:

— KSeF number is a unique number assigned to a specific e-invoice. Identifies the invoice in the KSeF system
— the bulk identifier applies to the shipment as a whole. Facilitates the identification and verification of whether invoices sent by the system have been accepted, rejected or waiting to be processed

When you encounter a bulk ID most often:

— when you send invoices in the KSeF system through a tool integrated with the KSeF API
— when you start to generate and send a larger number of structured invoices, and the KSeF system returns the number assigned for the entire application
— when you check receipts after an invoice has been issued and want to have a quick reference point for the entire operation, not just one invoice

Where it can usually be seen:

— in the data returned by the KSeF system in the shipping history or in the logs of the tool that issues e-invoices
— in the information related to the acceptance of invoices, especially when the system first shows the collective identifier, and only then each invoice receives a KSeF number
— in the official acknowledgement of receipt, if the tool saves and presents data from the shipping process

Important in the context of daily work and payment:

— the collective identifier does not replace the invoice number and does not replace the KSeF number of the invoice
— Aggregate ID is not what you type in the transfer. For payment of invoices most often it makes sense to have the invoice number assigned by the taxpayer, and the KSeF number in the transfer works if you consciously arrange the process in this way
— the bulk identifier is mainly used for technical identification of the shipment and quick status check when invoices in the system are still “going through” processing or integration

Invoice and lack of KSeF number: what it means in circulation

The absence of a KSeF number most often means one of two things. Either the invoice has not yet been accepted by the KSeF system, or it has not entered the National E-Invoice System at all in an effective manner. This is important because the KSeF number is a unique number that is automatically assigned by the system only after accepting the document. If there is no number, you are also not sure that the invoice is already in circulation as one of the structured invoices.

In practice, you can encounter such situations:

— the invoice was generated in the program, but it was not sent to the KSeF system. Then the document exists locally but has no status in the system
— the invoice has been sent, but the system has not yet processed it. Then the KSeF number of the invoice is not immediately visible
The system rejected the document. Then there is also a lack of a KSeF number, because the number is assigned automatically by the system only after acceptance
— someone looks at a PDF or a printout and expects the KSeF number to be there. Meanwhile, putting a KSeF number in the body of an invoice does not always work as people expect. You will probably check the KSeF number in the system and confirmations

The most important test is simple. If a structured invoice is recognized, then after the receipt of the invoice by the system, it should have its unique number. This number is generated and assigned to a specific document. This is the invoice identification in the system and the invoice identification number in the KSeF.

How to quickly check the number when you do not see it:

— enter the KSeF system or the KSeF taxpayer application and search the document by invoice data, for example by date, amount, counterparty or invoice number
— check if there is information about the receipt of the invoice. If there is, the assigned number should be available in the document details
— look at the official receipt certificate if you are using a solution that saves it. It is there that the identification is most often confirmed and that the document has been accepted
— if the number is still missing, go back to the shipping process. Sometimes the problem is that the document has been prepared, but the invoice has not been successfully sent by the system

It is also worth remembering the difference between the numbers:

— KSeF does not replace the invoice number. The invoice number assigned by the taxpayer continues to act as an element of numbering and internal order
— KSeF number is a unique identifier in the system. It is a unique character string that is automatically assigned by the system when the document is accepted
— if the KSeF number is missing, do not try to guess or rewrite it from an uncertain source. It is better to confirm the receipt and only then copy the KSeF number of the invoice

Payment and transfer: when does the KSeF number work in the transfer

Payment for invoices is the moment when many people wonder what number to enter, so that later it is easy to identify and quickly find the transaction. And here the important thing. The KSeF system does not impose a single correct description of the transfer, but in practice two approaches most often work.

— You enter the invoice number given by the taxpayer, because it is a numbering that everyone knows and that is visible on the document
— You enter the KSeF number in the transfer if you want to link the payment to the invoice in the KSeF system, especially when there are a lot of e-invoices in circulation and you want to have one unique identifier

It is worth remembering that the KSeF number is a unique number and is assigned automatically by the system after accepting the invoice. If you pay before the document has the status of acceptance and the assigned number, you will not enter in the transfer a number that does not already exist. Then, of course, the invoice number is better checked.

Important. In 2026, the KSeF number in the transfer title is primarily a matter of order and internal organization. Mandatory indication of the KSeF number in payments is to apply to payments made from 1 January 2027.

KSeF number in the title of the transfer: how to enter it legibly

If you decide to include the KSeF number in the transfer title, do so in a way that is legible and does not spoil the bank's search engines.

The simplest format:

— 'KSeF No: [KSeF invoice number] '
or
— 'KSeF: [KSeF invoice number] '

Good practices:

— copy the number from the KSeF system or from the application of the KSeF taxpayer, because the structure of the number is long and easy to typo
— do not add random characters in the middle of the number. KSeF is a unique string, so any error spoils the identification
— keep one, constant description convention. Then the identification of invoices and the identification of transactions in the bank is simply fast

Invoice payments: what to enter when the split payment mechanism applies

When the split payment mechanism comes in, the transfer has its own “message” and additional fields. In such situations, it is crucial that the payment is clearly assigned to the document.

What usually works best in practice:

— as a basis you enter the invoice number given by the taxpayer, because it is the number visible in the body of the invoice and in daily communication
— you treat the KSeF number in the transfer as an additional clarification, if you consciously arrange the process in this way and have it easily at hand after accepting the invoice

Important. KSeF does not replace the invoice number. Even if you use a KSeF number, the invoice number still becomes an important reference point in circulation.

Ultimately, for transfers under the split payment mechanism, the mandatory inclusion of the KSeF number is to apply to payments made from 1 January 2027.

How to describe a transaction so that it can be easily found in the bank

If you want to have a quick search by numbers, a simple scheme that combines both designations works.

— 'FV [invoice number] | KSeF [KSeF number] '
or
— 'Invoice [invoice number] | KSeF [invoice KSeF number] '

This approach works especially well when you have invoices in the KSeF system in larger numbers, and later you need to quickly match the transfer to a specific e-invoice.

If you use Altera.app, you can simplify it organizationally, because the app helps you organize invoices, monitor receivables and payment deadlines, as well as get back to the right document faster when you prepare a transfer.

summary

The KSeF number is a unique number that is automatically assigned by the KSeF system upon receipt of the invoice. It is used to identify the invoice in the system and is independent of what invoice number assigned by the taxpayer you have in your numbering.

The most important things to remember:

— KSeF does not replace the invoice number. On one e-invoice you can have both invoice number and invoice KSeF number at the same time
— The KSeF number is visible in the KSeF system and in the official acceptance certificate. This is where you will probably check the number if you do not see it in the body of the invoice
— The absence of a KSeF number usually means that the document has not yet been accepted in the National E-Invoice System or has not been successfully received
— When paying for invoices and describing the transfer, simplicity most often wins. You enter the invoice number, and when you want to link the payment to the invoice in the system, you add the KSeF number in the transfer or in the transfer title
— With a split payment mechanism, stick to a consistent description scheme so that transaction identification is quick even after a few months

If you want the KSeF number, invoice number, invoice acceptance status and access to the document in the system to be in one place, Altera.app helps you organize the flow of invoices and documents, monitor receivables and payment deadlines and return to the correct invoice faster, also when preparing a transfer and exporting transfer packages to the bank.