I've seen rail systems for sale, and a remark from someone wanting to get a rail system. What is the purpose(s) of that? Is a focusing rail for macro a different usage of the term (same as a focusing rack)?
I may have conflated it with rods, which are what I was wondering "what is that for?". I see them under Pro Video at B&H: a pair of long sticks under or on ether side of the camera.
As it turns out, what cameradojo.com’s article refers to a rails is sold at B&H as rods, with rails in their catalog being tracks for hanging lights from the ceiling. The short answer to my question is: a modular way to attach stuff to the camera, coming to SLR culture from pro video.