These are API functions that are not either asynchronous or synchronous. Typically, immediate APIs are those which do not communicate with a service or a physical device (or use the network in any other way) and are thus guaranteed to complete immediately, whether successfully or not. They are handled in two ways:
Processed entirely by the XFS Manager, which returns immediately to the application. Examples include WFSStartUp, and WFSSetBlockingHook.
Passed by the XFS Manager to the service provider as an immediate SPI. The service provider processes the request and immediately returns to the XFS Manager, which returns immediately to the application. Examples include WFSCancelAsyncRequest and WFMSetTraceLevel.