Is a normal servo, there is an electronics board that looks at both the signal coming in from the receiver, which is your means of telling it where to point, and the position sensing pot, which tells it where it is pointing. If they disagree, it tells the motor to drive. The motor drives its gearbox which in turn drives the pot until they agree. Co-incidentally, there is an arm attached to the gear driving the pot, which is what we are usually interested in.
In a winch, there is all the stuff mentioned above, but between the output gear that used to drive the pot directly and the winches pot, there is another gearbox. This slows down the drive to the pot so that for the same swept distance, the output shaft has to turn several times. In all of the early proportional winches, rather than a gearbox, a 10 turn pot was used. A normal pot generally uses the middle 1/3 f its sweep, 1/3 of 10 turns gives 3 and a bit, hence many winches were "three turn". The different turn winches available use different gearing between output gear and pot.
Programmable winches, OTOH, do it electronically, but do have the effect that if you reduce the travel, you also reduce the work done end to end. The motor doesn't magically produce more power to make up for the shortened run.
Using 20% of its available travel means that you have bought 5 times as much winch as you needed, assuming that it both works and survives. Reducing travel by programming is OK for small adjustments, but large adjustments keeping the same winch are better done by rearranging the string, paying close attention to the travel distance needed between the boom point and bridle ring, and their positions on the boom and over the deck. And whether or not to run the line through a doubling block to lose boom travel while gaining lots of torque.
