Those of us with long memories might recall the difference between the words 'channel' and 'function'.
Nowadays a 'channel' is thought of as a stream of information dedicated to a single task - such as working a rudder. That stream carries 'proportional' positioning information, and so can be thought of as carrying a set of theoretically infinite positions between 0% and 100%. If it was not used to position a servo, it might be used to control a 100-position switch, for instance!
In the early days of radio control, we did not have the luxury of multiple channels, each carrying proportional information, sent as a single frame which could be separated out and sent to different servos. We had a signal - it was either on or off. If we wanted to do more than one thing, we had to process the signal ourselves - usually mechanically. One 'channel' meant one signal.
In its basic form we used an 'escapement' - a bit of mechanics which switched its state once every time it was given a signal pulse. So one pulse would move a bi-directional mechanism from neutral to 'right', and the next pulse would move it to 'left'. For model boats quite complex switching systems could created, driven by a single channel. They were like old telephone dials, whereby a 10-position switch could be pulsed round one click at a time. if you combined that with a mechanical delay on each position you could select one action without initiating all the others - if you were quick!
More sophisticated kits had multiple channels - giving one switch for left, and another for right, a third for 'up' and so on. Full control of an aircraft needed 8 channels, and you needed some extra ones to give in-flight trim!
When proportional systems came out the old use of the word 'channel' became redundant - each proportional channel was the equivalent of an infinite number of 'single channels' - and for a while the word 'function' was used. So what would have been a 4-channel transmitter
became a 2-function transmitter
Nowadays we do everything electrically, but it would be possible to create a mechanical device
which would give Martin multiple switches
running off a single proportional 'function'....