Depending on the number of players and the system architecture, an MMORPG might be run on multiple separate servers, each representing an independent world, where players from one server cannot interact with those from another; World of Warcraft is a prominent example, with each separate server housing several thousand players. In many MMORPGs the number of players in one world is often limited to around a few thousand, but a notable example of the opposite is EVE Online, which accommodates several hundred thousand players on the same server, with over 60,000 playing simultaneously (June 2010[12]) at certain times. Some games allow characters to appear on any world, but not simultaneously (such as Seal Online: Evolution or Kolossium competition in Dofus); others limit each character to the world in which it was created. World of Warcraft has experimented with "cross-realm" (i.e. cross-server) interaction in player vs player "battlegrounds", using server clusters or "battlegroups" to co-ordinate players looking to participate in structured player vs player content such as the Warsong Gulch or Alterac Valley battlegrounds.[13] Additionally, patch 3.3, released on December 8, 2009, introduced a cross-realm "looking for group" system to help players form groups for instanced content (though not for open-world questing) from a larger pool of characters than their home server can necessarily provide.[14]