Conceptually and real life scenario - Proforma invoice is used to declare cost of goods to custom department to estimate duties/levies or to collect advance payment from the buyer. 
 
Proforma invoice is exactly the same as to Invoice.  The difference is Proforma invoice is need not to be paid and system does not transfer data to FI(Financial accounting).  No statistical data is created on the basis of Proforma invoice. 
 
F8 is delivery related proforma invoice and you can create proforma invoice for same delivery you as many as you like. Goods issue is not require to create Proforma invoice. 
 
In case of sample orders - (delivery free of charge- order type FD) - invoice is not generated.  In VOV8 - billing is not proposed.  Hence system does not generate invoice.  In Sample order/delivery account receivable does not increase but cost of sample order is added to sales promotion account or client suggested accounts. 
 
 
In summary:
 
Proforma invoice will not create the accounting entries. 
 
Requirement for Proforma invoice may differs from business perspective.
 
- One requirement is to intimate the customer to settle an advance payment. While making the payment, customer wants to see the sample bill for which he is going to make payment. So he demands proforma invoice. 
 
- Another requirement is to have a document while freight forwarders carry the materials. An invoice or Proforma invoice should be accompanied. This is the legal requirement. 
 
- Another requirement to have proforma while Stock Transport Order. 
 
 
Billing document will have their values triggers to accounts thru RAD resulting in an accounting document.
 
Proforma invoice will not triggers data to an accounts, so an accounting document will not get generated and it is mostly useful for Stock transfers from manufacturing plant to any depot whether locally or abroad.
 
It’s a copy that's all to show to during Custom when goods are in transit he has to carry this proforma document.
 
 
Difference between Billing and invoice.
 
Both are same in SD point of view:
 
- In SD terminology we call as Billing Document and
- In FI terminology we call as INVOICE.
- In MM again only INVOICE will be there for Vendors.

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