Prior to Patch 3.2.0 this setting was controled by the Dungeon difficulty setting to determine if a raid was to be a normal mode (10 man raid) or heroic mode (25 man raid).

All raids introduced before Patch 3.2.0 will use 10-player mode for the former normal mode (10 man raids) or 25-player mode for the the former heroic mode (25 man raids).

All newer raids begining with the Crusaders' Coliseum can use the 10-player heroic mode and 25-player heroic mode settings to access harder versions of the raids, not to be confused with hard mode events /actions within the same raids.

Patch changes

Patch 3.2.0 (04-Aug-2009): Intoduced.
            Prior to Patch 3.2.0 you had to use the Dungeon difficulty setting.

See also