Forgive my ignorance, I'm new to linear motion rails. Try as I might, after reading a ton of literature I cannot figure out why a rail would be described by the seller as "Z-axis". Specifically, a THK KR33 series rail. Couldn't a rail like this be used in any orientation, or is there some difference in a rail from the standard models that makes it more suitable for use as the Z-axis?
For instance, here's a completed auction for new THK KR33 rails described as Z-axis.
https://www.ebay.com/itm/THK-KR3310A...w/301839079698