What is it?
I haven't used it, but during the keynote the other day it seemed useful for remote team development.
So if a user has a project with your plugin in it, at build time you'd want Cloud Build to merge their custom defines from the Unity editor and their custom defines from Cloud Build with the defines in the .rsp file that you generate?
Anything defined in the Cloud Build UI would probably be final-say and trump the .rsp file, so you could setup multiple projects/targets with different configurations in Cloud Build, but pulling from the same source control.
Check out my game! https://www.ouya.tv/game/The-Adventures-of-Chris/
(The Adventures of Chris was greenlit on Steam - big revamp on the way!)