This is a little ambiguously worded. There are 3 RP realms in the North American region along with 6 clusters. All RP realms and clusters are cross-realmed together only for the purposes of battlegrounds. For PvE/trades/guilds/etc, players are only able to interact with others in their native realm or the realms to which they have been permanently linked via realm connections, effectively merged.
the bluepost detailing those permanent connections follows below:
Players on Old Blanchy and Atiesh can trade/guild/raid/etc with each other, but a player from Atiesh cluster won’t be able to interact with someone from Earthfury outside of a battleground.