Hi Josef
If you press F1 in that field, it explains in a very detailed way
I am pasting a gist below for you.. Most probably the doc you are seeing is a credit memo
In the SAP Standard System the field is used for:
a. Credit memos which refer to a particular invoice item
b. Subsequent invoices for an invoice item
c. Partial payments for an invoice item
d. Partial clearings of down payments.
In the first two cases mentioned, the terms of payment are copied from the cross-referenced invoice item into the item currently being processed. This ensures that items due on the same date are paid together by automatic payment. A special rule applies to credit memos which have a "V" in this field. The due date is determined in the same way asfor an invoice. If the field is empty (containing neither a
document number nor a "V") the due date is the baseline date for payment.
Br, Ajay M