Hi Rohit,
When it comes to PR and PO ideally the account determination should be automatic from either material master or service master from the MM configuration.
When it comes to NWA it is only cost activity when you need to know a primary Cost element (GL account), for internal NWA you need to know Activity Type (from CO master data setup).
Since PS is one of the most integrated modules, naturally you need a good knowledge of CO and ideally FI, MM, SD, IM.
Regards,
Paulo