According to this document, one of the regulations is about invoice form numbers and invoice numbers, specifically as follows:
– E-invoice form numbers contain a character with a natural number, that is 1, 2, 3, 4, 5, or 6, to determine the type of electronic invoice.
– Invoice numbers contain 6 characters, including both alphabets and numerals, describing the following information:
+ First character is one (01) letter – C or K: C denotes an e-invoice with the designated tax authority’s identification code; K denotes an e-invoice without identification code;
+ Two following characters are two (02) Arabic numerals denominated as an e-invoicing year that are two ending digits of the calendar year.
+ One succeeding character is one (01) letter in the prescribed set of T, D, L, M, N, B, G, or H which denotes the type of e-invoice in use.
+ Two ending characters are scripts that sellers may decide for themselves to meet their administrative needs.
For more details, please read the Official Dispatch No. 4144/TCT-CS issued on October 28, 2021.