Pure code search
Go inside the subroutine, this is the customized logic for this customized field.
How it define &configure at the front-end
Table from S001 to S500 is SAP Pre-defined and from S500 to S999 is Customized; Those table can be used as additional table source for report purpose, been updated by SAP or customized methods when system process/store standard tables like orders item table VBAP. (check this for more)
MC21 – Create Info Structure
Go to MC24/MC25:
Create and assigned the customized routine, and the routine number 989 is the same source place found by code search.