These looks a lot like voucher sequence prefixes.
Voucher sequences are not fixed in F&O so it's hard to tell; they are configurable.
However, there are a few strategies I have used to identify voucher types.
1: In voucher transactions, select an unknown voucher sequence, and use "Original Document":
Here I can see that the voucher number is related to a AR packing slip posting,
2: Find the number sequence, based on format (Can be filtered using advanced filter or sort), and then find the reference. You might need the help of an admin for this.
Here the voucher sequence is related to AP Invoices.
3: Check for a journal name.
Normally the GL journal name should is somewhat similar to the voucher sequence. Remember to check the voucher series under voucher number.