Hi
Its unfortunate that MM guy is dictating what should be PC. There is no rule Plant = PC.. It depends on business requirement
This is a very common requirement... Commonly addressed thriough Batch No...
If you Batch no concept, then have 1st letter of the batch different for each Mfg Plant and then based on that you can derive the PC in COPA..
May be you will need user exit at the time of SD Billing to derive the PC based on Batch No
Br, Ajay M