Revit works in its own coordinate system.
The easiest way to understand how base points work in Revit is to remember that:
THE WORLD REVOLVES AROUND REVIT.
The building you model in Revit is in a fixed position that can not be changed.
When you bring parts of the real world in, like surveys, CAD files other linked files, you move and rotate THEM around your Revit model. You do NOT move the Revit model.
Once the world is placed around the Revit model you tell Revit which way north is, and what real world coordiantes are at a chosen point (which can be any point).
For a very detailed explanation and practical advice refer to the attached Autodesk University class - Using Coordinate Systems in Revit.