What exactly are the pop caps for a layer? And extrapolating from that, what are the server pop thresholds roughly indicating (what makes a server low vs medium vs high?)
We’re not releasing specific numbers, that will likely change dramatically. Based on the response we got from name reservations, we’re anticipating large player populations for some realms initially. This may change as players switch to lower population realms. We’ll use layers to help improve performance in the short-term if large groups of players cluster in an area. We hope to reduce the use of layering over time as people spread out in each realm.
In case anyone was wondering about the context of this answer. It seems to me that they’re specifically referring to Phase 1, because Pazorax went on to say in another answer:
We’re absolutely committed to reaching one layer per realm by Phase 2.
It could mean that they’ll use layering in other phases for specific events that would cause massive numbers of players to be in the same area, such as the AQ events, but if that is indeed what they meant, then it’s highly unlikely they mean situations like city raids and Hillsbrad battles.
They’ve mentioned “turning off” layering for Phase 2, and they’ve talked about how even during Phase 1, they will be actively monitoring layers and reducing them as necessary to keep the world feeling alive. They have a lot of actual control over it as opposed to sharding, which is primarily or entirely controlled by the server. So it doesn’t appear to be something that would just trigger and move a bunch of people over without warning.
This is further supported by another post that pazorax made that goes into quite a bit of detail about layering and how it works, which I highly suggest everyone who is worried about or interested in it reads, which I will link at the end. But in summary, he explains that layering should only ever trigger if you’re invited to a group by someone in a separate layer (i.e., it won’t happen random just by changing zones or anything). Even if you logout, it now takes a few minutes for you to be removed from the layer you were in and placed in a new one, which both cuts down on potential abuse and makes things more persistent.
Another huge point is that not only is there a delay to going into a new layer, but if you’re currently engaged in or have recently engaged in PvP, there is a significantly longer delay.
All of this to say that although they probably just mean phase 1, even if they’re talking about using layers in specific situations beyond that, they’re making sure that layering does not interfere with world PvP. Even if you’re assigned to a new layer, it can’t move you over while you’re in combat or combat has recently ended (to give you time to loot), and if you’ve recently been in PvP combat, it has to wait significantly longer. Which means in something like a city raid or SS vs TM, you probably wouldn’t even be able to keep out of combat long enough for a layer change to occur.
As mentioned, here’s a direct link to pazorax’s comment explaining layering in the AMA:
https://old.reddit.com/r/classicwow/comments/ct08c7/welcome_to_the_rclassicwow_subreddit_ama_with_the/exi8loz/