Use of Profoma invoie in SAP GTS

Hello Expert,

I am new to GTS. Why profoma invoice required in SAP GTS not commercial Invoice.

Once profoma invoice has created then Customs document generating in SAP GTS. So what is the business logic.

After creation of commercial invoice do we need to create profoma invoice in R/3 ?

And where we do configuration setting for above points.

Appreciate if you can confirm at the earliest.

Thanks

Ravi.