The guy that works at my local P.O. said it was a way for the P.O. to track how fast a package takes to move through the system and helps to identify slow mail handlers and mail carriers. They hate it, and as someone said before, probably a way to identify what they consider non-productive employees or post office branches.

I asked and was told that there is no way that the sender could use that number as a "n/c" delivery tracking function.