The callback and local attributes can be applied as function attributes.
Callbacks are a special kind of remote call from server to client that executes as part of a conceptual single-execution thread. A callback is always issued in the context of a remote call (or callback) and is executed by the thread that issued the original remote call (or callback).
It is often desirable to place a local procedure declaration in the IDL file, since this is the logical place to describe interfaces to a package. The local attribute indicates that a procedure declaration is not actually a remote function, but a local procedure. The MIDL compiler does not generate any stubs for that function. For more information, see callback and local.