Protecting an Author (a User who has authored a Post or Page) protects all Permalinks leading to Posts (of any type) that were written by that Author.
This is where you create a new Restriction. There is a meta box here called Protected Authors where you can restrict content by particular Authors.
You can start typing in a name or just click in this box and a list of Users who have authored posts of any type will appear. You can select the Author whose content you want to protect from the drop-down list.
When you have finished creating Restrictions, click the Create Restriction button at the top-right of the page.