Are you referring to AI planes ?
The most important thing for AI planes is the wingspan compared against the parking radius. The plane will be parked on the smallest parking that has a radius bigger than wingspan / 2.
Only if several parkings will fit the plane, the atc_parking_codes of the parking becomes important.
So, if you want to affect the AI assignment, you must edit the scenery AFCAD with the (free) ADE or (payware) AFX programs, and work on both the parking radiuses and the atc parking codes of the parkings.