There are many internal tables needed to be checked when debuginng. The following are the main internal tables when posting like in T-code FB01.
Part 1 - Before udpate posting
1. XBKPF -> Collects the headers of the new documents to be posted
2.XBSEG -> Collects the items of the new documents to be posted.
3. For tax line items:T_BSEGT-> XBSEGT->BSEG->XBSEG-> at last update into BSEG/BSET…… with commit work
4.Other internal tables which are appended in Routine to be appended in Routine MODULE ZEILE_SPEICHERN -> FORM BUCHUNGSZEILE_MERKEN
5.YBSEG/D/C/T, YBKPF: Data from reference documents( exampleFBR2 in PAI MODULE REFERENZBELEG_LESEN. Of SAPMF05A)
6.XBSEG/C/D/T: Collects the items of the new documents to be posted.
7.SALTAB: For each company code (and one item for all company codes) total debit, total credit in all amounts (SAPFF014).
Part 2 - update
1.Append index tables like vendor
2.Insert BKPF
3. Insert BSEG
4. Insert into BSEG, BSI* and so on
5. DB Insert like BSIK & BSIS
6. After commit work, then update work process is triggered.