
R2 includes all of the updates included in Update Release 4 (and after installing will show Update Release 4 in the About Revit dialog), but also has added functionality available only to Autodesk Maintenance and Desktop Subscription customers. If you look on the Subscription Center, you will find Autodesk Revit 2015 R2 (released ). If you look on the Revit download page, you should see that the latest 2015 update available is Update Release 4 (released ). If you haven’t looked at the updates released for Revit (or Sundial), there could be some confusion between Revit 2015 R2 (Build: 20140905_0730), and Revit 2015 Update Release 2 (UR2) (Build: 20140323_1530). Click the following link for information on 39 Architectural, Structural, MEP, and multi-disciplinary enhancements made in R2:

As mentioned previously the Revit Sundial Release preview has ended and Autodesk Revit 2015 R2 is now available for Autodesk Maintenance and Desktop Subscription customers.
