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. 


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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.