Difference between Customer invoice, Free text invoice and Pro forma invoice in AX 2012

Pro forma invoice -

A pro forma invoice is an invoice that is prepared as an estimate of the actual invoice amounts before the invoice is posted. You can print a pro forma invoice for either a customer invoice for a sales order or for a free text invoice.

Customer invoice -

A customer invoice for a sales order is a document that an organization gives to a customer in response of a sale. You create such customer invoice based on a sales order, which consists order lines and item numbers. Item numbers are specified and posted in the ledger.

Free text invoice -

A free text invoice is not related to a sales order. It contains order lines that include ledger accounts, free-text descriptions, and a sales amount that you enter. You cannot enter an item number on this kind of invoice. You must enter the appropriate sales tax information.  Also, the customer balance is posted to the summary account from the posting profile that is used for the free text invoice.

In addition,  a free text invoice line can be used to sell a quantity of any kind of goods,services, or rights – providing unit price and quantity information to the customer. It also helps basic amount calculation, and it helps to better identify and explain charges to customers.