Is there a way to issue the command:
git checkout NameOfFile
on a modified and unstaged file from within MAGIT?
Reversing, reverting and discarding are distinct actions. Together with staging and unstaging, they are referred to as "apply variants".
To discard a change means to throw it away. Only un-committed changes can be discarded. When a staged change is discarded it is not only removed from the index (that is known as unstaging), but also from the work tree.
To reverse a change means to apply it to the work tree in reverse. Both committed and merely staged changes can be reversed. (Reversing staged changes is actually useful, i.e. when splitting up a commit).
Only commits can be reverted, in other regards it's similar to reversing.
See https://github.com/magit/magit/wiki/History-Manipulation for details.