If XYZ depends ONLY on the values in ABC and BCD, then instead of clearing XYZ the following code can be used:
*XDIM_MEMBERSET CATEGORY = PLAN
*XDIM_MEMBERSET CURRENCY = USD
*WHEN FLOW
*IS F20
*WHEN ACCOUNT
*IS ABC,BCD
*REC (EXPRESSION = %VALUE%,ACCOUNT=XYZ,FLOW=F99)
*ENDWHEN
*ENDWHEN
Due to accumulation XYZ=ABC+BCD
And stop using COMMIT, useless operator with WHEN/ENDWHEN!
Vadim