A reversible clip can be unscrewed and remounted in more than one position, letting the same knife carry left or right handed and sometimes tip-up or tip-down. A four-position clip offers all four combinations.
Two-position clips usually swap sides, so left-handed and right-handed users get the same experience. Four-position clips add tip-up and tip-down on each side.
The knife needs threaded holes machined at every position it supports, which is why four-position clips appear on knives designed for it rather than as an aftermarket addition.
It is a two-minute job with the correct Torx driver, usually T6 or T8. Take the screws out one at a time, keep them together, and do not overtighten them going back in. The screws are small and the threads are in the handle material.
A drop of removable thread locker stops clip screws backing out, which is one of the most common ways people lose a clip.