Threat table
The list every MMO enemy keeps of how much it hates each player — and which decides who it hits.
Whenever a player damages the enemy, heals someone in the fight or uses a taunt, their value in that enemy's table goes up. The enemy attacks whoever is on top. That makes combat legible: a tank generates lots of threat through dedicated abilities and soaks the hits, while healers and damage dealers stay under the ceiling. EverQuest (1999) called it the hate list, and World of Warcraft (2004) made the rules famous — another player has to exceed 110 percent of the tank's threat in melee range, or 130 percent at range, to pull aggro.
The margin exists so the enemy doesn't ping-pong between two players with nearly identical values. The whole classic tank–healer–DPS trinity rests, in practice, on a sorted list with some hysteresis.