This sounds like a really bad literal translation by a non-English speaker - trying to understand the meaning is frustrating for us also! I guess it is meant to say, "If any window is open on top, it will not process". So you're probably using something like Microsoft Windows, and "it" is not the open window, but the program running underneath. An open window on top will stop the program running. Does that make sense? It's just my guess! :) Electronics manuals are an excellent source of terrible translations. It sounds like someone looked up each word in a dictionary but didn't write the whole sentence properly.