A MassZone must have a level as its bottom to be occupiable;
If it does not, it is still a MassZone, but it is not occupiable
Namespace:
Autodesk.Revit.DB.Analysis
Assembly:
RevitAPI
(in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0)
Since:
2012