Use of Ctr Key: PM02, for subcontracting.

Dear Experts,

I am having a confusion regarding subcontracting. I know the use of Ctr Key: PM03, for external service. But what is the exact use of Crt Key: PM02?. If we use PM02 than PR can be create with item category  "L", and user can add materials which are need to send for subcontracting. Can any one please give me clarification on PM02 ctr key.

Regards,

Anil.