Command & Conquer: The Covert Operations

The Covert Operations is an add-on for Command & Conquer featuring 15 new missions and several new music tracks and multiplayer maps. Unlike the original game, the missions of Covert Operations can be played at any time and in any order, but are not accompanied by mission briefing cutscenes. The add-on added two new units to the game, and the 15 new missions are more difficult than the campaigns of the original. The add-on pack also features the DOS version's soundtrack, which includes music that was absent from the Windows 95 (or Gold Edition) version. Covert Operations was originally needed to unlock secret missions, which were later enabled by a patch to the main game

Source: Wikipedia, Command & Conquer: Tiberian series#The Covert Operations , available under the CC-BY-SA License.