Selecting the Purchase Orders tab or the Purchase Order Number field on the Account Overview screen allows you to define purchase order numbers for the account and for each plan instance on the account:
To display the purchase order information on the Account Overview screen, you must enable it using the appropriate configuration settings.
Getting Here: Click Configuration > Billing > Invoice Settings
Note: If the Purchase Orders tab does not display for you after enabling the Show Purchase Order Number client parameter, then navigate to Configuration > Security > Account Access > Account Overview > Pages and turn on the display of this functionality by selecting the corresponding radio button.