You may have to provide additional background information to your questions. Depending on the previous experiences of people reading your question, there can be different interpretations, and without a more detailed context, you may not get the response that you want.
From a purely technical perspective, any groups of metadata are available / usable globally in a system, regardless of the site / structure a piece of content resides in, or the user that interacts with that content. There may be UI customisations that limit the specific data entry fields or data display forms a user can access depending on the context (site / group membership). Most of metadata definition and UI customisation is typically done by developers, not administrators. There is only a very limited functionality available in the default user interface that allows administrators to make very simplistic changes to usable metadata, and that functionality should typically only be used in evaluations as it has very problematic impacts on long-term maintenance / management of metadata models.