Claiming is a game mechanic in many MMORPGs, including both Final Fantasy XI and Final Fantasy XIV, that prevents an enemy being fought by a player or group from being attacked by other players and prevents other players from receiving its Item drops.
Final Fantasy XIEdit
Claim status in Final Fantasy XI is indicated by the color of a monster's name. Enemies with an yellowish off-white colored name are unclaimed and can be attacked by anyone. Red names are claimed by you or your party. Purple names are claimed by someone else not in your party. Orange names are monsters that some unfortunate player has called for help on, and are subject to attack by anyone but yield no rewards for their defeat. (Green names indicate NPC status, which are not attackable by players.) GMs will not recognize any claims on a monster that are not enforced by the game mechanics, hence the player expression "yellow (or white)'s fair game".
To claim an enemy in Final Fantasy XI, it is necessary to perform an Enmity-generating action upon it. Merely being aggroed by it is insufficient. Thus if competitively Camping monsters, it is necessary to bring along a means of doing something hostile that is fast and has a long range--too slow, and you'll be beaten to the claim; too short, and you will have to run closer to claim, letting someone else have a chance at it before you.
Only one monster can kept claimed by an individual at a time, and if another monster is claimed the first will go unclaimed. Claim is also lost when everyone who the monster has enmity for is dead or in another zone. Monsters outside their normal area that are not being actively fought will also go unclaimed before losing aggro and returning to their normal area. Unclaimed monsters with no damage over time effects active will also start to regenerate any lost HP at a rate of 10% every few seconds.
There is also a bug of years-long standing that may cause claimed monsters to spontaneously go unclaimed even while being actively fought in some cases; to avoid this, it is recommended that at least one person have the monster as their auto-attack target at all times, even if they do not actually have any attention of hitting it (turning around may be necessary). It is also possible for a player outside the party to steal claim by rising to the top of the monster's hate list; this usually requires that player to cast curing or enhancing effects on the players who are fighting it and actually have claim.
When a monster dies, its item drops go into the treasure pool of the party that had claim upon it, or into the inventory of the individual that had claim upon it if they are not in a party. Players should beware of letting a monster go unclaimed with damage over time effects active on it! If it expires while unclaimed, its dropped items and Experience points will be lost to oblivion.
Because claiming a monster means claiming its loot as well, claims over rare Notorious Monsters are a source of great conflict and drama among the players. For many years, the three "land kings" (Behemoth and King Behemoth, Fafnir and Nidhogg aka "Fafhogg" collectively, and Adamantoise and Aspidochelone) which only popped once every 21-24 hours were monopolized by groups using claimbots to claim them in the fraction of a second that they spawned before honest players could even see them; these monsters "spawned purple". Eventually, to counter this, Square Enix implemented a mandatory delay between when a monster spawned and when it could be claimed, and made it semi-random to make contested claiming more a matter of luck. During the interval before a monster can be claimed, attempts to attack it will simply fail without explanation.