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The CDO library is a hierarchical library consisting of objects and collections. Throughout this hierarchy, you can create children objects from their parent objects, thus creating the same collection for different objects. While these duplicated collections might have the same name, the information accessed by each one is specific to the object to which each refers.
In the CDO library, the Session object is the highest-level parent of all the other objects and contains all objects and collections. This makes sense, because you must have some type of session, either an Exchange Server session or an offline session, to start accessing information stored in an Exchange Server database.
For additional information about CDO, refer to http://msdn.microsoft.com/en-us/library/ms527038.aspx in the Microsoft® Developer Network (MSDN™) Library.