A Table Right or Column Right with Scope = "Background only" gives the holder only background access on that table or column.
Background access consists only of:
•Data access by a constraint,
•Data access by a relationship attribute,
both as an indirect result of some foreground access initiated by the holder. The Delete Rule and Update Rule relationship attributes require SELECT background access if set to "Restricted" and UPDATE background access if set to "Cascading" or "Nullify".
Scope = "Background only" allows you to hide a table or column fully from a user, while at the same time allowing constraints and relationships to manipulate that table or column on behalf of the user.
It only makes sense to give a holder "Background only" scope on a table or column B if that holder has some foreground access to some table or column A that indirectly requires access to B.