When you use PM02 control key for External Procurement of Services, Service Material group & Cost elements can be given in PM configuration which will be defaulted in PM Orders.
Mostly, Material group for External Service will only one but Cost element (G/L Account) will be different based on the type of service company hired from vendor.
As per the business requirement, maximum data for External Procurement of Services can be given at configuration level so that user entry or interference or mistakes can be avoided.