title_body: What is the background of remote controller modules? I've seen that in many remote controllers you can plug in an external module under it.
  
  
  Are there other purposes for these modules than just being transceivers?
  Are the module bays offered by multiple brands, or is it just a single brand thing?
  Are there different styles of modules that look similar but don't work interchangeably?
upvoted_answer: There are multiple purposes for the pins on the back. One of them is to connect an external transmitter module like crossfire, which gives you a longer range.
  
  Another use is to flash a receiver with another firmware by connecting it with a servo cable (on FrSky transmitters at least) to the bottom three pins.
  
  There are transmitters from several different manufacturers, and I think some of them are compatible with each other, but you shouldn't trust that all of them are.