There are a number of ways to enter AP invoices in AX2012 and it depends on what amount of data you want to capture or what process you want to put in place. Prior to AX2012 you have the ability to invoice a purchase order or you could use the invoice journal, invoice approval journal etc. Invoicing a PO worked well for item based purchased but the journal approaches worked well for managing at and account and summary level there wasn’t a middle ground for non-PO based invoices if you wanted to enter more details and use distribute the lines. With the ability to have category based purchases you also can put in put details for later spend analysis.
This video gives a quick overview of non-PO based invoices.
This video was recorded with AX2012 R2 but the basic concepts apply to any release of AX2012.