Standard fields in split criteria

Dear gurus,

is there any way (apart from debugging) to find out which fields SAP uses as split criteria for SD-bills?

In our production system, items keep getting split into different bills. Split analysis says this is due to different values in the field unloading point in customer order. Apart from that we would like to have those items in one bill, our test system does not split similar order items.

BTW: Here is the coding of the routine assigned  in VBRK/VBRP field in Copy control. No trace of unloading point:

* Additional split criteria
DATA: BEGIN OF ZUK,
MODUL(3) VALUE '001',
VTWEG LIKE VBAK-VTWEG,
SPART LIKE VBAK-SPART,
END OF ZUK.
ZUK-SPART = VBAK-SPART.
ZUK-VTWEG = VBAK-VTWEG.
VBRK-ZUKRI = ZUK.

Now we would like to check the split criteria any ideas? Or are the standard fields hard-coded?

Cheers

Alicia