What tables make up the Master Posting window (Tools->Routines->Mater Posting)? I cannot seem to find the information at all.
I think the Master Posting window mainly uses the Posting Definitions Master table (SY00500).
Search resource descriptions by Windows and you can find the master posting window under the Company series. From there you can find all the technical table names used by the master posting window.
Yes you are right
Table SY00500 under company database.