I've noticed that the number of reviews required to remove a post from a queue varies. For example, to accept/reject a suggested edit, you need two votes of the same kind. If the votes are conflicting, the number goes higher. This seems to be the case for the other queues as well. (Additionally, I think that a mod-review is binding)
So, my question is: what are the criteria for a review to be marked as a "completed" review? Also, when is a review considered invalidated (i.e. "no longer reviewable")?
Formerly When is a post removed from a review queue?.