Hi Ricardo,
COPA means -Controlling and profitability analysis
Standard SAP doesn't have this functionality and we have to implement OSS note.
The PAOBJNR is determined by looking up the combination of characteristics in table CE4xxxx'. So when system posts the line item to CE1 it will check if for the combination of chars contained in the line item a PAOBJNR already exists in CE4XXX.
Other option is use VBFA table to link sales order and credit/debit memo.
Thanks,
Rau