None of the answers above are accurate at least in my scenario. I had a revision decision, and after I submitted the revision, the status went to "Under review", and after about 4 weeks, it has now changed to "Awaiting reviewer scores". In this case, the reviewers were already lined up to get the revision, and so it doesn't make sense for "awaiting reviewer scores" to just mean that the reviewing is in progress -- in fact, it's "under review" that means that, and it doesn't make sense for "under review" to mean pre-screening. What the "awaiting reviewer scores" most plausibly means here is that the reviews are now due! 4 weeks is also the time I'd expect the AE to allot for the reviewers (from past experience), and so the timing is right for the status to change from "Under review" to "Awaiting reviewer scores" - so it just means some reviewers haven't yet submitted it and the reviews are either due or overdue. This explanation also makes sense if you just look at the English of the status "Under review" and "Awaiting reviewer scores" -- the scores aren't awaited unless it's due! It's probably why they chose this language for the status message. In addition, for all my submissions in the past, the "Under review" status has always meant that the paper was actually with the reviewers as opposed to with the AE waiting for the assignment; papers have been in the "Under review" status for me for several months after which they change to "Awaiting AE recommendation". So it makes no sense that "Under review" means pre-screening (as suggested by one of the comments) - it may be different for different journals but I doubt that is the case for any journal.