I dont know the exact definitions, I looked up the terminology up in my manual. But the essence for me is:
An encoder is a turning knob that has no beginning and end. It is usually used in relative mode, which means it gives a +1 or a -1 depending on the direction it is turned. (or higher numbers to indicate speed in turning) It has noticable "ticks" when turning the knob, every tick is a +1 or -1.
As example, i use one to do beatjumps in traktor: every tick is a beatjump of 4 beats, so I can fast forward into the track (and back) easily without using the mouse.
a potentiometer had a beginning and end, and usually outputs values in the intervals [1,127] or [-64,+64]. And is for some types of controls more suited imo, like controlling the fx knobs in traktor.
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