If you were instead looking for the used in the "Vault of Secrets" (a secret area in the game) to unlock secret levels without spending mana orbs, here are the relevant codes:
(Note: In the Lite version, the text often reads: "More levels in the full version!") geometry dash free levels
Themed collections of levels (like the Fire Gauntlet or Demon Gauntlet) that offer rewards upon completion. If you were instead looking for the used