Wow, it’s more complicated than I thought:
“The coordinate system or Revit has been rehashed many times on blogs and posts, but has been clear as mud for me, and I suspect a lot of other people. So I took the time to really figure out it, and here’s a 30 minute explanation of how Revit’s coordinate systems work, interrelate, how true north works, and how to put multiple copies of a building on a site.”

Another good article on this on AECbytes: Understanding Shared Positioning in Revit