All transmitters have an address coding switch. The code set there more or less represents the key to your transmitter. Transmitter and receiver will only work together if they are both set to the same address.
The transmitter buttons are known as channels. Three such combinations of actuated transmitter button (channel) and individually set transmitter address can be programmed in the receiver modules of type FE3 in order to trigger a certain switching function there. Several receivers can also be grouped and centrally activated in this way!