In trx VTFA (if your billing is sales order based) choose your billing type
and SO type, there select your item categories and there select the field VBRK/VBRP
data. In that field you will see the currently used routine. With the help of
your ABAP guy create a copy of that routine under a different number and add
your lines of code. Let's say you use routine 001.
* Header data
* VBRK-xxxxx = ............
* Item data
* VBRP-xxxxx = ............
* Additional split criteria
DATA: BEGIN OF ZUK,
MODUL(3) VALUE '001',
VTWEG LIKE VBAK-VTWEG,
SPART LIKE VBAK-SPART,
END OF ZUK.