ADN Revit API Training Labs including Xtra.
This repository contains the source code and Visual Studio solution of the ADN Revit API Training Labs including the old historical Xtra samples.
The official collection excluding the Xtra labs is also available from the Autodesk Developer Network ADN Revit Developer page, in the section for Samples and Documentation.
It lives in its own ADN DevTech RevitTrainingMaterial GitHub repository.
If you have no need for the Xtra labs or do not know what they are, you can simply stick with the official version provided there.