The slate is what the projector shows whenever there is no media on it: blacked out, nothing taken yet, or closed for the day.
Under Settings, The wall it can be black, a colour, or a logo. A chosen logo is copied into the app, so moving or deleting the original cannot leave the wall broken mid-show.
Blackout is also stop: it cuts the wall to the slate and pauses whatever was playing, so releasing it picks up on the same frame. The silent-audio warning, for a file whose sound nothing can decode, shows on the wall's own strip in the control window, never on the wall.