Giving you a IFAET, and IFATC perspective.
I’m not currently on my PC so I cannot provide imagery, but when we are editing an airport, we make sure we add all the frequencies possible. Unicom, Ground, Tower, Approach, and Departure. There are more that aren’t currently in IF like Centre, and Clearance Delivery, hence why there currently is 2 ground frequencies, Clearance Delivery is the frequency that operates around the gates, clearing people for pushback and stuff. Anyhow, we put all the frequencies into the editing software, where they all get added into the game. Only 1 person can control each frequency that the airport offers on the Training Server, and only CTAF/Unicom is used on Casual.
The Expert Server has all these features unlocked. So depending on how many frequencies the airport in question has, the amount of stations we can open!
Atlanta only has 2 approach frequencies, hence why IFATC have opened up Radar frequencies of nearby airports.
So, if you really wanted to you could be an idiot and open all frequencies at a stupidly large airport. Don’t do this lol.