Officially it seems to be stated that it shards your realm cluster into a PvP state, however I see many people from other clusters than my own.
Is Warmode just one giant shard consisting of all players with it active regardless of server? (I know RP servers are separated).
There can be multiple shards. It depends on the population in an area.
As more people enter an area, the game will make a new shard, any player who enters the area after that, will be placed in the new shard.
I’m fairly sure it is across realm clusters too, I’m on Khaz’Goroth, but I see lots of Frostmourne players in War Mode, even though Frostmourne isn’t connected to my realm and I don’t see them in cities.
It’s all a method of balancing server loads. So no one server becomes overloaded and lags out.
And yes, RP realms have their own shards that are separate from the rest.
I think it used to be more realm cluster oriented earlier in the expansion. Now you can run into just about anyone.
WM is awful, and needs to go. Bring back PvP servers.
In general: If you’re on a non-RP server, you will be in non-RP server war mode. If you’re on a RP server, you’ll be in RP server war mode. (Note that this doesn’t necessarily preclude you from joining the other server type’s version of war mode, since an RP server player can be invited to a group by a non-RP server player and end up on non-RP server war mode, and vice versa…)
This can have some serious ramifications where you go from casually stomping every alliance you come across in non-RP server war mode… to getting stomped by two full 40 man raid groups of alliance when you get invited to RP server war mode. (Those Moon Guard alliance players are FERAL…)