What is the intended functionality for Substitute Performance Document Evaluators in All-in-one?

Summary:

We are trying to understand the expected functionality of the Substitute Performance Document Evaluator in all-in-one evaluations. Our working assumption is that this would give a manager's manager the ability to evaluate the performance for a team two levels down on behalf of the line manager. So for example, director (A) evaluates the performance for the employees (C) of a manager (B) that reports to the director (A). (A evaluates C instead of B evaluating C).

Is there any other expected functionality. Should/could HR be able to do this for a manager via My Client Groups using All-in-one Evaluations? Perhaps a manager's admin?