Clarify how to allow/disallow an unbound function in $filter/$orderby

11.5.4.1 Invoking a Function
states (emphasize mine)

Functions can be used within $filter or $orderby system query options. Such functions can be bound to a resource, as described above, or called directly by specifying the namespace- (or alias-) qualified function name.

It states how bound functions can be used.
The standard also explains how function imports can be used but there seems to be now explanation if and how they are connected for the "called directly" case.

What is necessary to use a function in $filter an $apply? (e.g. is a function import necessary?)
And depending on the answer, how can it's usage in these cases be restricted/disabled ?