Inside The Windows Messaging System (1993)

The five categories are:

Every window in the system is associated with a particular application message queue. Because it contains most of the data used by the Windows messaging system, think of the queue as a sort of command center linking a window handle to a particular task, and serving as the keeper of the status bits vital to GetMessage()/PeekMessage(). Only QS_POSTEVENT messages exist in the application message queue; other message types imply messages synthesized by the system.

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