Hallo Herr Berliner,
In case of account based COPA derivation takes place only once i.e. during sales order creation. SPART cannot be changed by derivation in standard at the PA segment level.
In case of costing based COPA derivation happens twice; once during sales order creation if customized or at the PA segment of the billing document. Derivation takes place again during creation of the COPA document i.e. record type F. During creation of the COPA document it is possible to change chars like SPART via derivation.
If you want to change the standard behavior i.e. derive SPART at the PA segment level then you need note 638097 to modify the standard behavior. But this has some side effects like KE396 and you have to follow the instructions in the note.
danke und gruss
Waman