Hi,
AWTYP (Reference Transaction) is a field where posts automatically while posting the Accounting document number.
There are different kind of Reference Transaction are there.
1. From FI side Accounting document BKPF
2. From MM side Material document MKPF
3. From SD side Billing document VBRK
Those are standard SAP Reference Transactions, system itself updates that field, we can't change those types.
Logically Reference Transaction is as same as Header Table
Ravi Polampalli