IID 

The IID structure is a GUID14AUUS3 structure defined by MAPI that identifies a MAPI interface.

Remarks

Interface identifiers are used to specify the interface to use to access an object. For example, in a call to IMAPISession::OpenEntry to open a folder, a client would specify IID_IMAPIFolder in the lpInterface parameter.

All of the IID structures for the MAPI interfaces are defined in the MAPIGUID.H header file.