The term sky hook is also used in architecture to describe how a light fixture or interior facet is to be held when there has not yet been anything determined that can do so. It's a sort of catch all in blueprints used as an excuse for something that needs to be in the design, either by command of the person(s) for whom the building is being drawn and/or the firm creating the blueprints but has yet to be given a physical place to be affixed. Kind of like, as in my business ( auto body work), when we need a part but the part isn't given a name per se, we just call it a left-handed smoke shifter. One of those inter-office jokes that shows how often you are working and thinking in a language that is unique to the environment in which it is used.