Invoice Lines list the items to be purchased. Before an Invoice is generated, these items are listed as Sales Order Lines in a Sales Order. Once payment is made, Receipt Lines and Transaction Lines are created.
Invoices are created upon closing the Sales Order. When the Sales Order is posted, the General Ledger will reflect a debit to Accounts Receivable and a credit to the Revenue Account. Receipt of payment on an invoiced Sales Order will debit the Deposit Account and credit Accounts Receivable.
See Creating Invoices to learn how to create Invoices.
SeeProforma Invoices for information on creating purchase orders that do not impact your Accounts Receivable.