USPS - Delivery Point:
(1) A single mailbox or other place to which mail is delivered. A street address does not necessarily represent a single delivery point because a street address such as one for an apartment building may have several delivery points. (2) A specific set of digits between 00 and 99 assigned to every address that is combined with the ZIP+4 code to provide a unique identifier for every delivery address. The DP is encoded within the POSTNET or Intelligent Mail barcode.
Edit. As an aside, I think current Microsoft word still calculates it that way when you "bookmark" and address and insert a barcode field. At least it doesn't seem to be sending a query out to the USPS.