Halo 2 multiplayer ranks
From Halopedia, the Halo wiki
The ranking system in Halo 2 is the numeric representation of a player's multiplayer experience. An identical system based off the Halo 2 rank system is also employed in the competitive playlists for Halo: The Master Chief Collection.
Halo 2 uses a skill-based matchmaking system, informed by a numeric value of 1-50 to represent a given player's skill level in any given playlist. When matching players for online games, the algorithms in question try and pair up similarly-skilled players to ensure roughly evenly-matched games. The displayed 1-50 rank is an abstraction of an XP value, hidden to the player and calculated behind-the-scenes. Each game completed awards or removes XP, depending on the result of the match - this means that poor performance in a game can result in losing rank, while winning results in players ranking up. Ranks are calculated purely on wins and losses, meaning actual in-game performance such as KDR, Flag captures, bomb plants, hills captured etc do not affect the final result; merely being on the team does.
Rank is calculated per-playlist, meaning that a player's performance in one playlist does not affect their rank in another.
Once a game is completed, the game awards or removes XP from each player depending on the final result. This is achieved by consulting a table (shown below), dependant on the difference in level. In broad terms, winning or losing matches against similarly-ranked players nets a lower XP gain/loss, whereas winning matches against players with higher skill gaps will gain greater rewards or greater loss. For a given player in the game, the table is consulted to perform a matchup against every single player of an enemy team in the game - if the player has completed a match of 8v8 slayer, the table would be consulted to determine the XP gain/loss for all eight opposing players. Once these values are tallied, the total is divided by the amount of enemy players, to gain a final XP value for the game as a whole.
To give an example of this system in use, consider a free-for-all match in which there are four players;
To calculate Player A's total XP gain, the table must be consulted three times - once for each player they matched up again. In this case, they won against a player with a three level difference, a two level difference and a five level difference. Thus, it can be determined that Player A earned 79, 85 and 70 XP - respectively. This value is then summed to get a total of 234, before divided by three to get an average value of +74XP. This calculation is done for every player to determine each player's XP gain/loss.
With the player's total XP values known, the game can display a single rank number as a representative of their overall skill. Ranks are indicated by a 1-50 value, with a new player holding rank 1 and the highest-ranked players holding rank 50, with most players averaging around rank 10. Each rank has a simple icon alongside its number, with the final seven ranks holding a unique symbol - a nod to Bungie's love for the number. Each rank has an XP bracket assigned (shown below). However, in order to prevent frustration at constantly flipping between two ranks if games are repeatedly won and lost, players will keep their 1-50 rank value even if they drop below the original requirements to achieve it, and won't de-rank until they hit the mid-point of the previous rank's requirement. For example, if a player needs 10,000XP to hit a new rank, achieves 10,000XP but then loses a game and drops below the 10,000XP total, they won't revert back to the previous level until their XP total hits 5,000XP.
Rank XP requirements
The 1-50 rank held by a given player determines which players they will match up with in multiplayer matchmaking, by aiming to match players up with similarly-ranked players. The matchup requirements are listed below.
Rank XP gain modifiers
With the systems listed above, one last consideration is the XP modifiers given by rank. In order to help new players in being able to level up, lower-ranked players are not penalised as hard when losing matches while they are low-leveled. This % value peters off as they level up and by rank 29, is no longer in effect. However, Halo 2's online XP is a zero-sum system, meaning that there is a set amount of XP points available to the entire player population as a whole, and the raw XP gain/loss systems ensure when a player loses against another player, the winning player gains the same amount of XP as lost by the loser. In order to maintain this system, the higher-ranked players begin seeing the opposite affect applied to their wins. Upon reaching rank 42, players will begin earning a lower percentage of the XP they would otherwise earn, with this penalty increasing per-rank, capping out at max-rank players at level 50 only receiving 50% of the XP they earn for a win.
A rank is recorded for every playlist an individual plays in, with the exception of non-ranked gametypes and custom gametypes. There were leader boards for the majority of the playlists, however Bungie removed them because of cheating and hacking problems.