ZPS stands for Zero Preload System, these days it's also called P-zero. Generally it was used as a ride height feature. It allows lowering of the ATV via the preload collar while not letting the springs float loose. On some models it becomes possible to run a longer shock while still getting a proper ride height. By using the P-zero spring in this manner it also gives a very soft ride in the small small bumps. There are even more tuning possibilities with ZPS, but they will be used if the quad and use of the quad require it.
ZPS shocks are only build for instances that require it (this will be defined after filling in the build sheet).