Distribution model filter for ORDRSP

Hi;

Can someone please explain why message type ORDRSP only have one standard filter (KUNNR – Customer) available in BD64, but in BD59 2x entries exist:

KUNNR  E1EDK01               1              RECIPNT_NO       212         10

VBELN   E1EDK01               1              BELNR                   83           35

Also, they are both defined in BD95:

KUNNR  KNB1     KUNNR

VBELN   VBUP     VBELN

Why is VBELN not available as filter in BD64?

To my knowledge, above entries were supplied by SAP – i.e. SAP standard.

Thanks