I'm having difficulty finding someone to help out on this post. You linked an SR ID that appears to be Pega PRPC based and not Robotics. Though you are asking about MDI which is Robotics. Did you link this SR by mistake? Could you clarify please what your question is so that I can reach out to the appropriate Subject Matter Experts to help you out?
Are you asking about the Signal Component? If so, this is a component that can be used to wait (calling the Wait method on the component) for something else to happen on a separate thread (calling SignalAll or SignalOne). I would urge you to check the help for specific questions on it.
In the case of MDI Windows, you can't really use the WaitForCreate method of a new window since it will require a key (which won't exist yet). You could use a Signal component where you have one automation that starts with the MDI window Created event and calls SignalOne. In your automation where you are waiting on the new screen, instead of calling WaitForCreate, you could call the Wait method of the Signal component.