There are a number of ways to get an accounts payable invoice in the system which approach you use will depend on the business process you are implementing. For example if you want to do expense type invoices where you don’t have a purchase order you can use a vendor invoice.
The advantage is that if you have PO invoices or want to use lines items that are either item based or category based then you can have the same process for all types. As well you could add functionality like the budget check, workflow etc.
Here is a quick overview.
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