Instance Bosses are bosses who spawn in an instance. These instances are tuned for raid-groups and vary in both level and difficulty.
Before the Burning Crusade expansion, the endgame raiding content was tuned to level 60 and to 40-man raidgroups. With the Burning Crusade expansion there came new endgame raiding content, tuned for level 70 players in 10- or 25-man raidgroups. Since all of these bosses require more then 5 players to defeat them, they are called raid bosses.
Therefore we separate the Instance Bosses into two categories: