Battlefields are located throughout Final Fantasy XI. They are an "instance" based battle where a single player or group will enter a special arena and fight a predetermined event, without fear of interruption from other players.
Most battlefields are accessed through objects called Burning Circles, which has led to battlefields as a whole being referred to as Burning Circle Notorious Monster (BCNM) events, even if said battlefield does not involve a Burning Circle.
Different battlefields have different rules. Some battlefields will only allow 1 person to enter, others will allow up to 18 players to join in the form of an "Alliance". Generally, though, the majority of battlefields allow a full party or a half party, up to 3 people, to enter. It is always acceptable to try to do a battlefield with less than the maximum number of people required, but obviously this almost always makes the encounter more challenging.
Most battlefields have an artificial level cap placed on a character when they enter. Since there is an overall limit cap on the game, you might also want to assume all of them have a set maximum level for participants. When a player enters a level capped area, he will take on the statistics of his job for that level. He will also be limited to equipment appropriate for characters of that level.
If a character enters a battlefield with a level under the level cap on the zone, he will be considered his current level. The level cap will not bring him up to meet the recommended level for the encounter.
A character will not actually lose any experience just by entering the battlefield because of the level cap. In areas where a level cap is in place on a character, should the character die, the experience point penalty for dying is applied as appropriate to the capped level or their current actual level, whichever experience penalty would be lower.
All battlefields which are not part of a quest or mission require a special item to access. For the most part, these items are Orbs, which can be acquired by trading Beastmen's Seals or Kindred's Seals. Other battlefields, such as the Limbus battlefields, require much rarer items, or even items acquired from a different battlefield. For example, in order to access the 4th Floor of the Temenos battlefield, the player must have items which can only be obtained in the lower floors of Temenos.
- Most battlefields are not actually instanced, even though they appear to be. Instead, a number of copies (usually three) exist of the actual battlefields, the monsters in them, and the reward crate; these can be viewed in the game's data files with the proper tools, and cheating tools can be used to warp into another group's battlefield. If all three battlefield copies are in use, no other group can participate, and they must wait in the small un-duplicated 'lobby' portion of the zone or leave.
- Until it was fixed, it used to be possible for dishonest players to exploit the ability access other players' battlefields to wait for them to defeat their foes, then steal the contents of their reward crate.
- The battlefield behind the hut in Ghelsba Outpost doesn't even have the pseudo-instancing described above. The quest to find an Adventuring Fellow requires the player to fight in this battlefield; when the quest was first released, players queued up in lines dozens long to enter the battlefield.