Yes, on missions starting from an aircraft carrier, the start position is determined by the ship's DP file.
Ie:
[flight_deck_section]
width=32.577024
length=262.128
takeoff_start=-2,16.7801544,-119
takeoff_stop=-2,16.7801544,120
landing_start=0,500,5000
landing_touch_down=1.279999,16.7801544,-126.0524784
landing_stop=1.279999,16.7801544,0
On land airfields, the start position is in the airbases.dat file under the airfield you wish to take off from:
[runway.0]
id=9000
name=Abbeville
heading=130
allegiance=0
units_across=1
base_lat=N50 8.66
base_lon=E1 50.35
base_alt=201
takeoff_start_pos=-500,0
takeoff_stop_pos=1600,200
landing_start_pos=5000,350
landing_touchdown_pos=500,0
landing_stop_pos=-250,0