title_body: What is the background of remote controller modules?
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.