At the end of the last year (2021) Portuguese government announced that implementing ATCUD (Validation-Sequential number code) to invoice and other fiscal documents will be mandatory from January 2023 however for QR code we cannot say the same thing. QR code must be added to invoices from January 2022.
ATCUD contain of two parts;
- Series validation code which come from government and,
- The sequential number of the document within the series which is added by the taxpayer.
You can find more information here.