You can only get Revit Systems as part of Revit Series - Systems Plus...
In other words, you license Revit Systems + Autodesk Building Systems, but can not license Revit Systems by itself.
You can, however, install only Revit Systems if you desire.
HTH,
Martin Schmid
Autodesk Consulting
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.