Default Document Type, Posting Keys and Transaction Codes
T-CODE OBU1 TO KNOW DEFAULT DOCUMENT TYPES AND POSTING KEYS
F-32 = FB1D
F-44 = FB1K
F-03=FB1S
We can also view in SE93
To know how document types is picking autoamatically for the f-03/f-44/f-32
T-CODE = OBXH UMBUCHNG