The ArcGIS Explorer 900 API has five new namespaces, namely:
Contains all the classes necessary to interact with the application.
Contains all the classes necessary to interact with the map.
Contains all the classes necessary to represent the geometry of features and spatial referencing.
Contains all the classes necessary to represent and query data.
ESRI.ArcGISExplorer.Threading—contains all the classes that enable you to develop multithreaded customizations for ArcGIS Explorer.
The Data, Geometry, Mapping, and Threading namespaces are contained in the assembly ESRI.ArcGISExplorer.dll, and the Application namespace is contained in the ESRI.ArcGISExplorer.Application.dll assembly.