Master data look up

Hi,

In the update rules, I need to do a conditional master data look up and hence cannot use 'Master data attribute of..'. I was wondering if I need to look into the P table or M table, are there any reasons to look up from any specific table. I do know that M table is just a view master data tables. My object is a NOT time dependant.

Any help would be greatly appreciated.

Thanks,

Ace