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KSeF and the New Rules for Identifying Transaction Parties on Invoices
- Paweł Gorzelec
- Jan 23
- 3 min read
From 1 February 2026, amended rules for issuing invoices will come into force, based on the Regulation of the Minister of Finance and Economy of 7 December 2025. Although the amendment formally concerns only one provision of the invoicing regulation, in practice it introduces a fundamental change in how transaction parties are identified, particularly in the context of the mandatory National e-Invoicing System (KSeF).
The purpose of the amendment is to fully organise identification data used in business transactions and adapt it to the realities of mandatory e-invoicing. An invoice is no longer merely a descriptive document – it becomes a component of the tax infrastructure, subject to automated and real-time technical verification.
Which invoices are covered by the new rules?
The new identification requirements do not apply solely to invoices issued directly within KSeF.
They cover:
all structured invoices,
invoices issued in offline or emergency modes,
invoices issued outside KSeF but delivered to the counterparty before being sent to the system.
In practice, this means that every invoice that will be submitted to KSeF – or is intended to be submitted – must comply with the new identification rules at the moment of issuance, regardless of the technical mode used.
A new identification data model – what does this mean in practice?
From 2026, simply “entering a tax number” will no longer be sufficient. The legislator introduces a multi-level identification model, depending on the tax status of the seller and the buyer.
Seller’s data
The invoice must include the number used by the seller for tax purposes, namely:
the tax identification number (NIP) – for domestic entities,
the tax identification number issued in the country of establishment – for foreign entities.
This number becomes the primary verification key in KSeF and cannot be omitted or replaced with other data, such as the company name or address.
Buyer’s data – general rule
As a rule, the invoice must indicate the number used to identify the buyer for tax or VAT purposes, depending on their tax status:
NIP – if the buyer is identified for domestic tax purposes,
VAT number – if the buyer is identified for value added tax purposes,
another tax identification number – if the buyer does not hold a VAT number or a domestic tax ID.
Buyers without tax registration
If the buyer is a taxable person not registered for tax purposes, or a legal person not being a taxable person and not registered, the invoice should include their tax identification number – if such a number exists.
Failure to include this data may lead to issues with the correct processing of the invoice in KSeF.
SME procedure – a new identifier instead of NIP
The most significant change concerns entities using the SME procedure, a special EU exemption for small businesses conducting cross-border sales.
In this case:
the traditional NIP is not used as the main identifier,
the invoice must include the individual identification number assigned under the SME procedure.
This requires technical and procedural adjustments, particularly for businesses operating across EU markets.
Why are these changes so important?
From 2026 onwards, identification data on invoices will no longer be a mere formality. It will be:
technically verified by KSeF,
used to ensure data consistency in economic transactions,
crucial for the correct circulation of invoices and tax settlements.
Incorrect or improperly selected identifiers may result in invoice processing issues, rejections, or the need for corrections.
Practical tip
To prepare safely for the new rules, businesses should already:
update contractor verification procedures,
adapt accounting and financial systems to new identifiers,
train sales and accounting teams,
develop a pre-KSeF invoice data verification checklist.
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