Depending on the type of Transmitter you have more or less channels to send commands. To be able to maneuver a multicopter you need at least 4 channels for throttle (up/down), rudder (yaw), elevator (forward/backward) and aileron (left/right). Some flight controllers have optional settings like a flight mode that you can change in flight. For this you need an additional channel. You might even want to have even more channels to control the shutter of a camera, switch lights or drop something.

So even for a simple multicopter you probably want to go a little bigger and invest in a transmitter that has at least 6 channels - if not 9.

Transmitters are pre-programmed for a specific setup or mode. Mode 2 is a common one where the left stick controls the throttle and the yaw and the right stick sets elevator and aileron. Usually you can change the mode by opening the transmitter and switching the gimbals that hold the sticks - if you really have to.

Computer controlled transmitters allow you to program special settings. For example they can change the output of the sticks so you have less movement the closer you are to the center and making movements near the end of the stick more sensitive (and twitchy) to give you the full range of your control. This is called “Expo”, because the output is based on an exponential curve.

Also there is “mixing” of multiple channels. This allows you to give one input and the transmitter sends data to more than one channel. You won’t need this for a multirotor as the flight controller will do it all, but it might be handy if you also wish to fly a fixed wing plane or glider too.

The receiver on the other end of this wireless link passes the input on to the flight controller.

It’s important that transmitter and receiver use the same protocol as in “speak the same language”. Most manufacturers only support their very own protocol. If transmitter and receiver match, you still have to make them know each other. This is called binding. Check your manuals on how to do this.

To make things more difficult, expensive setups can have the receiver send data back to the transmitter. This is used to display or log telemetry data from sensors like battery voltage, GPS data, etc. on your transmitter.

Also the receiver output has to be compatible to the flight controller you want to use. There are various protocols like PPM, PCM, and SBus and most FC need a specific input protocol.