Cross-realm zones

Cross-realm zones (colloquially known as CRZ) is a game design tool created to "fix the issue of having a ton of players at max-level, but lower level zones being dead." CRZ operates by putting players from multiple realms into the same "instance" of a particular zone to make it "feel active and alive."

Currently, cross-realm zoning only occurs between realms within +/-3 hours time zone difference.

Zones unaffected by CRZ
Certain zones that are not affected by the feature, including:
 * Wintergrasp
 * Vault of Archavon
 * Tol Barad
 * Baradin Hold

Known issues

 * When leaving a major city into a cross-realm zone (such as Stormwind to Elwynn) in a passenger mount such as the Reins of the Traveler's Tundra Mammoth with a player passenger on board, the player passenger will be dismounted (suddenly placed on the ground, standing) and the NPCs returned to their positions on board.

Cross-Realm groups
With the exception of zones that are never cross-realm (notably Stormwind and Orgrimmar but only on RP realms), generally all members of a cross-realm group will be phased to occupy the same copy of a zone. The rules governing which copy of the zone the group sees are complicated. Roughly the selection operates as explained below.
 * In the following rules, home zone refers to the copy of the zone that a character would see while ungrouped.
 * Generally all characters on one realm or group of connected realms share a home zone.
 * Due to cross-realm zones some characters from different (unconnected) realms will also share a home zone.
 * A newly-created group (ie with a single member) starts by using the leader's home zone.
 * From this point forward, group leadership no longer affects phasing.
 * Phasing to a different copy of a zone may only be triggered by a change in group membership (a member joining or leaving).
 * If one group member exceeds the level of anyone else in group, the group will phase to his home zone.
 * If two or more group members with different home zones are tied for maximum level, the copy housing the most max-level characters is used.
 * In the case of a tie in the previous rule, the zone copy housing the most characters of any level (but ignoring level 1 toons) is used for the group.
 * If there is still a tie, no phasing occurs and the group remains on the most recently-selected copy of the zone.
 * The Online/Offline status of group members does not affect phasing decisions.