Views:
Select the order in an Invoice Manager or In a New Invoice window, when ready click “+ Process Invoice”

 
Explanation: Processing an invoice updates the appropriate general ledger accounts, posts any sales tax liability to the appropriate sales tax vendor, and posts any commission liability to the appropriate salesperson’s account. The invoice number will be assigned at this point.

SAVE vs. PROCESS INVOICE:

When clicking Save rather than Process Invoice, if you have NOT previously processed the invoice (no invoice#), it will just save your changes. 

If you had previously Processed the invoice, then clicking Save will do the same thing that Process will (reverse out all transactions and post new transactions).