I couldn't really find much help for scripting spawning aircraft into parking spaces hot. So I decided to document what I have found for others. I have only tested this on the Nevada Map. For other maps your results may vary.
First take note of an airbase in mission editor. When you zoom in you will see the particular parking spaces listed for each airfield on the map.
Of these parking spaces not every aircraft type can park in every space. This is true in mission editor and also true via scripting. You can use the mission editor to find available spaces for a particular aircraft type by creating an aircraft type take of from parking hot. There is a drop down of available spaces.
There are two parking variables available for the groupData ["units"]. These are ["parking"] and ["parking_id"] both can be used for parking in a desired space. The friendlier one is ["parking_id"]. Use it like this:
["parking_id"] = "G20"
The value for ["parking_id"] uses the same value that is printed on the map in the Mission Editor. If the aircraft can be park in the space in Mission Editor, you can park in that space via scripting when you assign ["parking_id"] the same value.
The other variable ["parking"] is numeric and matches a ["parking_id"] as printed on the map in mission editor. For instance the variable ["parking_id"] = "G20" , might be the same parking space as ["parking"] = 185 . Use either one of these variables or both. The variable ["parking"] seems to take precedence if you include both. You can find these ["parking"] numbers by parking an aircraft in the mission editor in the desired space, saving, and then opening the mission script file inside the .miz file to find the value.
A couple of things that tripped me up which might help you.
Spawing Incrementally.
Don't even bother trying to spawn a group of similar aircraft by incrementing the variable ["parking"] + 1. This fails because the ["parking"] spaces on a given airbase probably are not sequential for a given type of aircraft. If you do this, unexpected behavior will result. The way I solved spawning a group of same type aircraft is to create a table of known available parking spaces and increment over it to spawn the aircraft:
parkingMap = { "E13" , "03" , "02" , "01" , "06" , "F163" }
["parking_id"] = parkingMap[1] , etc
Only One Time
I have found you can only spawn aircraft into a given parking space one time. For instance if you created the above table parkingMap, then you decided to iterate over that table multiple times to repeatedly spawn aircraft. This will fail because you cannot keep spawning into ["parking_id"] = parkingMap[1] multiple times. If you do this, you will experience the next concept.
The Main Ramp
If you spawn an aircraft into the same space twice or use a ["parking_id"] or ["parking"] value that does not exist, the aircraft will spawn into the parking space which I am calling the "Main Ramp". Every airbase has a Main Ramp and its location is different on each airbase. Every aircraft "seems" to be able to spawn to the Main Ramp on every airbase regardless of the length of the runway. I found some airbases with Main Ramps that are actual ["parking_id"] spaces which you can also spawn repeatedly into. Other Main Ramps might just be the approach end of the runway or a taxiway.
You can use the Main Ramp to spawn aircraft repeatedly into the same spot. For example if you were to iterate over the above table parkingMap a second time, every subsequent spawn will be to the Main Ramp. This behavior happens regardless of what value you assign to ["parking_id"] when you attempt to spawn a second time in the same space.
Stacking Aircraft.
If nothing is blocking their way aircraft will start to move 20 second after spawning. If you stack aircraft on the Main Ramp or something is blocking their way as they spawn, aircraft won't queue up and pull out one by one. If you stack multiple spawns on the Main Ramp, as soon as an aircraft moves there will be a crash. Remaining spawns will just stack up and nobody moves, just like ORD.
Only So Many
Some aircraft types have a lot of available parking spaces, others not so many. Too many waiting or taxing aircraft will just gum up the works. Eventually nobody will move, just like ORD.
Hope that helps!