Originally Posted by FBinNY

Imagine either derailleur as a car parallel parked between two others on a hill.

The two cars are like limit screws. They determine the length of the space you have to maneuver in, and can be mover to give you more or less range of travel. But moving them won't move your car. You have to do that yourself. You can put your car anyplace between either car, and if you release the brake your car will roll back until it touches the lower car.

The derailleurs are the same, the cable sets their position within the working range according to the lever position, and when the cable is slackened the derailleur (not a car anymore) will settle against the return spring limit (usually outer on the rear and inner on the front). When the cable is pulled all the way tight the derailleur will stop against the upper limit.

In most systems the return spring limit is used to set that gear position, (inner front, outer rear) with the lever detents (clicks) setting the position in all the others. In some cases the high position is full up against the upper limit, in others, just shy of it allowing a bit of overshift which then settles off the limit.