Lee Swordy -- author of AFCAD -- says it best in the AFCAD Readme file:

35. Parking Assignments for AI Aircraft
Flight Simulator permits assigning airlines and other aviation groups to use specific parking spots at an airport.  It also allows assigning general classes of aircraft such as cargo, GA, and military to parking spots.  And although it doesn't directly allow you to assign specific aircraft models such as 737s to specific parking spots, you can designate parking spots to be used by certain sizes of aircraft. This section deals with the rules and parameters used to make the AI park where you want at an airport.

Parking Criteria:

FS uses the following criteria to decide where to park an AI aircraft.  These criteria are in order from most important to least important:

1. Parking Codes   (KLM, AAL, etc.)
2. Parking Types   (Gate, Ramp, Cargo, etc.)
3. Parking Areas   ( Parking, Gate 1, Gate A 1)
4. Parking Radius   (31m, 38m, etc.)
5. Parking List order  (first, second, third parking spots listed etc).

Read more after putting AF2 Readme.htm on your desktop

bill