What does “AD” mean in a small purchase voucher payment handling field?

There are three options for a small purchase voucher’s payment handling:

• NH: Normal Handling
• PU: Pick Up
  These checks come back to the Controller’s Office for departmental pick-up. Proper identification is required at time of pick-up.
• AD: Additional Documentation Required
  These checks will be returned to the Controller’s Office and the uploaded invoice/documents will be printed and mailed along with the check to the vendor.