Sustainable DPS
While some bosses have a strategy, often this strategy can be escaped or overcome through alternative means unintended by the developers. One is the burn-down (some bosses are actually designed this way). The other way is being able to withstand a boss's attacks and continue to produce damage indefinitely. In general, sustainable DPS refers to the ability to produce damage over a long period of time through their attack chain. In a raid specifically, this refers to the ability to stand and fight however long it takes until the boss is defeated.
Achieving sustainable DPS in a boss fight can be challenging; energy/mana, consumables, ammo and chaos control can all be serious barriers to the ability to stage a long-term fight. Raid bosses also often cull inattentive players, making extended vigilance a problem as well. Since the burn-down fights are faster and more convenient, they are often preferred, saving sustainable DPS as a last resort.
However, this method allows a specially geared and prepared raid group to face bosses with very high health values, making it a valuable tool in the raider's arsenal.
Read more about this topic: Damage Per Second