How does the Require PO function work?

On the Terms/Sales tab of the Edit Customer screen, there is a field called "Require PO" under the Options section.

When you toggle this field on, it activates a condition where if you create a quote, order, or invoice for that customer, it will require a purchase order number to be typed into the "PO" field on those forms in order to save.

This condition is helpful if your company has customers that require a purchase order number to order or to pay bills.