Invoice Document type not linked with a customer or vendor?

Hello Every Body,

I have a serious problem with a definition of type of invoice number on Header Level that has no:

- Customer Defined

- Vendor Defined

in BKPF for example the docment type "AB" or "SA" are = G/L account document

But i have to communicate to the client JUST the 4 cases:

0: The invoice is customer invoice

1: The invoice is Vendor invoice

2: The invoice is customer Credit invoice

3: The invoice is Vendor Credit invoice

The G/L account document document type..., where can i group hem in 0 ,1, 2 or 3 ?

Or in wish criteria i have to look to determine exactly the document type of invoice number.

*Bseg-SHKZG Credit / Debit is not interesting solution, this is on item level/ i need for Header Level.

*Bseg-KOART Account Type, is always empty voor invoice type = G/L account document, So, is also not a good idea...

*Switching from BKPF and VBRK via AWTYP (=Ref. Transactn) is not always working, so not a very good solution...

Is there any skills/ways in FI or ABAP to determine exactly the document type like that AB, SA G/L account document of that 0,1,2 or 3 ?

Thank u very much for replaying.

Ouail