From discussion in CRUC-98:

Michael Armstrong added a comment
So the implementation in version 4.2 seems to be lacking a state. If Al make a comment and marks it as needing resolution. Then Betty implements it and marks it as resolved. When Al comes back in, if he doesn't like the resolution, he can reopen it, but what happens if he likes the resolution. How does he indicate that he reviewed the change and likes it? Does he have to make a new comment? Seems like another state of approved or verified or reviewed or some such would be useful.

Marek Parfianowicz added a comment
Hi Michael, thanks for your feedback.
Let me ask a question:
why Al can't just leave the comment as is (i.e. in the resolved state) if he likes the resolution?
I'm asking because:
new commits added to a review are marked as unread, so you can easily find new content added
new comments as well as changes in the resolution state are marked as unread, so you can easily find them too
So:
"resolved" + "unread" = fix waiting a review
"resolved" + "read" = fix is approved
"resolved" switch to> "needs resolution" = fix is rejected

Michael Armstrong added a comment
Maybe some of this is my background. We are a company coming from a much more formal review processes, so I'm used to having a very positive action that lets a reviewer accept or reject an implementation.
This issue I have with relying on the read or unread state of the comment is that the implementer can't see the reviewer's read/unread status. The implementer has no way to know if the reviewer hasn't seen the implementation yet or saw it an approved it.