The Infinite Flight Airport Editing Team (IFAET) make sure they add all the frequencies possible. Unicom, Ground, Tower, Approach, and Departure. There are more that aren’t currently in Infinite Flight like Centre, and Clearance Delivery.
For example, take KATL. There currently is two ground frequencies, Clearance Delivery is the frequency that operates around the gates, clearing people for pushback and stuff. The IFAET put all the frequencies into the editing software, where they all get added into the simulator. Only one 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 they can open.
Atlanta only has two approach frequencies, hence why IFATC have opened up Radar frequencies of nearby airports.
To add on to that, in the ATIS, expert server controllers can include frequencies located at other airports if we need more radar frequencies.
This allows for greater flexibility and the ability to handle high level of traffic effectively as a team.
Explanation partly provided by Ecoops123, an Airport Editor