pUnkOuter parameter of IClassFactory::CreateInstance method

What's the reason behind the pUnkOuter pa开发者_JAVA技巧rameter of IClassFactory::CreateInstance method?

HRESULT CreateInstance(
  [in]   IUnknown *pUnkOuter,
  [in]   REFIID riid,
  [out]  void **ppvObject
);

The MSDN's description of pUnkOuter parameter notes:

If the object is being created as part of an aggregate, specify a pointer to the controlling IUnknown interface of the aggregate. Otherwise, this parameter must be NULL.

Can you good people please enlighten me on what is this parameter for?

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See Aggregation in COM fundamentals. If you create an instance of COM object which is a part of an aggregate, you have to supply so called "outer IUnknown", which is owner COM object interface.