Tier 6 refers to the set of class equipment that is acquired from the bosses of Hyjal Summit, Black Temple and Sunwell Plateau. These sets are meant as a follow-up to the Tier 5 equipment found in Serpentshrine Cavern and Tempest Keep. In Patch 2.4, these sets that originally consisted of 5 pieces were extended to 8-piece sets by drops found in Sunwell Plateau; however, the set bonuses are still acquired at the 2- and 4-piece levels.
Tier 6 also refers to guilds working on the bosses in the aforementioned raid instances.
Druid: Thunderheart Regalia (Balance), Thunderheart Raiment (Resto), Thunderheart Harness (Feral)
Hunter: Gronnstalker's Armor
Mage: Tempest Regalia
Paladin: Lightbringer Raiment (Holy), Lightbringer Armor (Protection), Lightbringer Battlegear (Retribution)
Priest: Absolution Regalia (Holy), Vestments of Absolution (Shadow)
Rogue: Slayer's Armor
Shaman: Skyshatter Regalia (Elemental), Skyshatter Raiment (Resto), Skyshatter Harness (Enhancement)
Warlock: Malefic Raiment
Warrior: Onslaught Armor (Protection), Onslaught Battlegear (Fury)
Tokens that drop from these bosses are one of the following: