all this CRZ technology was introduced during cataclysm if i remember correctly, so its been like 5 years now. it has always been confusing to me.
u can play with friends from other servers, like do quests together or even dungeons.
but can you raid crossrealm ? i play on stormscale EU server and unfortunately there are no polish guilds, so i would like to join one from burning legion server and raid together with them. is that possible ? are there any requirements, conditions or its just not possible ?