25-player mode refers to the former dungeon difficulty setting (heroic mode) for raids, Patch 3.2.0 introduced the new interface (raid difficulty) setting. This setting is accessed from the raid leader's portrait menu; currently the only other difficulty settings are 10-player mode, 10-player heroic mode and 25-player heroic mode for raids. Patch 3.2.0 did away with the old reference, other than showing up in achievements.

See also