..and if so, what is the requirement?
I couldn't see it listed on
/privileges, particularly on
/privileges/comment which seemed the most likely candidate, and I couldn't find any other posts here on meta...
I would assume that because the above blurb from the comment privilege mentions voting, that you can vote on comments when you can comment. Since comment votes are intended to reduce "+1 good point" type comments this assumption makes sense, but I don't know for sure.
The page doesn't mention that you can vote on comments, but it doesn't mention that you can comment either. The privilege pages really just describe functionality and it's just the link that describes what you can do. Maybe "comment everywhere" should be changed to "use comments everywhere"?
If we can treat the FAQ remarks as general:
Do I have to log in or create an account ?
It doesn't specifically mention comments, but it's a privilege reserved for registered users, and as pointed out in the comments (by the OP) it requires some reputation.
It is a privilege, but not a privilege distinct from normal flagging. Note that the Vote up privilege mentions it:
As pydave notes, the help page for the commenting privilege makes no mention of voting, so it should be assumed the ability to vote on comments is distinct from the ability to create them.