Purchased orders can be entered with two main line types based on an item or based on a category. With the new UI in AX7 there are a few ways to access the new purchase order form one being the workspace purchase order preparation or from the procurement and sourcing menu.
Creating a category based purchase order is good for expense type purchases where you might want a purchase order for later matching with the invoice but have a need to track inventory with an item.
Here is a quick overview.
AX7 Build 7.0.4230.16130