The COM objects of OPC Data Client are based upon the underlying .NET objects. In order to avoid duplicities and mismatches, the documentation is primarily maintained for the .NET objects. The .NET objects are exposed using the "interop" mechanism to the COM world using an automated translation provided by Microsoft. Therefore, a documentation that applies to a .NET type or member that is exposed to COM can be assumed to apply to the corresponding .NET type or member as well.
The bulk of the reference documentation for COM type libraries is generated from the type libraries themselves. This means that only a limited descriptive text (typically, one line) is available with each type or member. You need to look to the reference documentation for the .NET assemblies in order to find the more detailed documentation.
In addition, some languages or COM-based tools do not make direct use of the type libraries, and therefore require some additional effort to use the COM objects - such as knowing their ProgIDs, dealing with interface IDs (IIDs), etc.
For more information, see COM Type Libraries Reference.