This Geospatial API namespace contains all of the Mg* classes plus AcMapMap and AcMapServiceFactory.
The reference is the Autodesk.Map.Platform.dll. Help for the classes prefixed with Ac can be found in the AutoCAD Map 3D .NET API Supplement Reference (<Map 3D root folder>\Help\sdk.ref.net.supp.chm). Help for the classes prefixed with Mg can be found in the Geospatial Platform .NET Reference (<Map 3D root folder>\Help\sdk.ref.gis.platform.chm).