When one process invokes another via Remote Procedure Invocation (50), the call is synchronous, so there is no confusion about which call produced a given result. But Messaging (53) is asynchronous, so from the caller’s point of view, it makes the call, and then sometime later a result appears. The caller may not even remember making the request, or it may have made so many requests that it no longer knows which one this is the result for. Now, when the caller finally gets the result, it may not know what to do with it, which defeats the purpose of making the call in the first place.
