The National Electronic Invoicing System (KSeF) is a teleinformatics system for the management of electronic invoice flows, managed by the Director of National Tax Administration.
The purpose of the KSeF is to issue, receive, store structured invoices, access structured invoices and mark structured invoices with the identification number assigned in the KSeF.
A structured invoice is considered to be issued on the day it is sent into the National Electronic Invoicing System.

Manages XML files of structured invoices, according to the logical structure of the FA(2) e-invoice, and notifies rejection of structured invoices or inability to issue them. It acts as a virtual postman for receiving, formally checking and sorting structured invoices to the addressees.

XML files of structured invoices are transmitted via communication channels to the KSeF. The system performs a formal check of the files and, if the structured invoice is correct with the logical structure of the electronic invoice FA(2), notifies the date and time of issue of the structured invoice and the structured invoice identification number assigned in the National Electronic Invoicing System, enters the invoices in the restricted area of the Invoice Portal and forwards them to the recipient. If the structured invoice does not conform to the logical structure of the FA(2) e-invoice, KSeF will notify the date and time of the rejection of the invoice or the inability to issue the structured invoice, if the National Electronic Invoicing System is not available.

XML files, i.e. structured invoice files, can be:
1.prepared directly in the taxpayer’s financial and accounting programmes and sent to KSeF via API;
2. sent using the free tools provided by the Ministry of Finance: the taxpayer’s KSeF application; or
3. micro-entrepreneurs have access to the functionality offered by the Ministry of Finance through the e-Mikrofirma web application.

Currently, entrepreneurs can use the KSeF system on a voluntary basis.
During the optional period, structured invoices function on the market as one of the permitted forms of documenting transactions, alongside paper invoices and electronic invoices currently on the market.
Currently, the receipt of a structured invoice via KSeF is possible after the recipient has accepted this method of receiving the invoice.
If the structured invoice is not sent to KSeF, it is considered not to have been issued.
The issue of structured invoices is not conditional on acceptance by the recipient. A structured invoice is considered to have been received by its purchaser, via KSeF, when KSeF assigns it a number which identifies it in the system.