The using A for B; directive is active for the current scope, which is limited to a contract for now but will be lifted to the global scope later, so that by including a module, its data types including library functions are available without having to add further code.
{ ... }pairs delimit the scopes. Also, it can be understood that a function forms a scope. I guess a contract forms a scope as well. In this case forusing A for Bdirective, it is unclear to me if "the current scope" can be a function scope.