WPBP function processes all splits from IT 0, 1, 7, 8, 27, etc. The splits are due to changes in these infotype records. Output: WPBP table with all splits (splits per split period), IT table with infotype 8 w/t entries for each split. The split types are like work cetre, basic pay, CC, etc.
see link below for more info.
Understanding functions in Payroll Schemas (specific eg, WPBP)
You do not need to use SETIN op'n for w/tr's from IT 8. In a pcr called by PIT, if you add to a w/t other than the one from IT e.g. ADDWT 4556, then this creates an entry (with the same splits as in IT) for this w/t in OT (which is moved to OT), unless you remove the split before ADDWT 4556.
Split can be removed with op'n ELIMI and then reset to previous values with op'n RESET.
My proposal earlier is for w/t's entered in IT 14, which is a requirement in many companies.