Contracts may have purchase orders. When they do, the purchase order (PO) tab collects and manages the POs.  Contract may have more than one purchase order.

The navigation button, located at the bottom of the screen changes depending on the mode of viewing.  Add mode will present the navigation in a wizard format.  The user must visit each tab in order.  In update mode, the user can visit any tab at any time. 

Purchase Order tab