Pro Forma numbers are used when in your workflow you need to create invoices that might requiere to be amended. You want to keep these invoices as preliminary versions, without an invoice number until there is a confirm invoice ready to be sent.
But you still need to keep record of this preliminary invoices and Pro Forma numbers give you an internal number that can not be changed but will help you to identify each of this documents.
Once you know which invoice is the one approved you can locate it with it's Pro Forma number and move it to the Invoice Manager, where it will be set as send and will get an invoice number.
You can either erase or ignore the other versions of the invoices as it won't have any effect on the final version.
To activate the Pro Forma numbers you need to access the server setup and go to the financial Tab. There you will find the setting "Use Pro Forma Numbers" set it to YES, together with the setting "Give invoice number when create" set to NO.
Each time you create a new invoice it will be given a Pro Forma number (READ only) that you won't be able to change.
Only when moved to the invoiced manager the invoice will get an invoice number and the pro forma one will disappear.