What does the invoice number look like and what do the digits mean?

Our document numbers always follow the same structure:
Two letters to indicate the document type, the respective billing period in the format YYYYMM as well as a random serial number at the end.
The first letter indicates the type of document (G: goods/supply delivery, S: service, D: difference amount), the second letter is either an I for an invoice or a C for a credit note.
Ex. GC-202008-123 is a supply delivery credit note for August 2020.