I was told by my dealer that 10-1 is is retail order. Most of us will just get 10's as priority. To get a higher number, it has to be changed by the regional rep. 1 is the best you can get/moves you to the front.
20 would be as 50sonic said, its labeled as a "stock" order.
0-9 can only be assigned by a regional rep or higher ups. 10-19 is a retail order, and 20+ is stock. 99 I believe is to "order" a car but not have ford pull the order for build as I understand. Just to have it in the computer
There's a lot of helpful information in the Wiki, including a discussion on priority codes. Take a look here. The Wiki can be accessed via the link at the top menu bar.