A program that issues a call and does not regain control until the call completes cannot perform any other operations. This type of operation, referred to as blocking, is not suited to a server application designed to handle multiple requests from many clients. Asynchronous call completion returns the initial call immediately so the application can continue with other processes.
Microsoft® SNA Server uses the RegisterWindowsMessage function for asynchronous support. With "WinAsyncAPPC" as the input string, an application passes a window handle by which it can be notified of verb completion. The application then issues the verb. When the verb completes, a message is posted to the window handle that was passed, notifying the application that the verb is complete.
With the exception of asynchronous RECEIVE_AND_WAIT , MC_RECEIVE_AND_WAIT, RECEIVE_AND_POST, and MC_RECEIVE_AND_POST, which can issue certain other verbs while pending, a conversation can have only one incomplete operation at any time.