Special Stage Key
From Sonic Retro
The Special Stage Key (スペシャルステージキー), also known as the Special Key (スペシャルキー), is a collectible item that appears in Sonic Heroes and Sonic Advance 3. This item allows entry to a Special Stage when collected.
Throughout Sonic Heroes, Special Stage Keys are found within cages that are placed in several locations in each level. Once the player has collected a key, they must bring it to the end of a level without letting the leader drop their Rings from damage or lose a life (such as by falling into a bottomless pit), otherwise they will lose the key and will have to find another one. If the leader has a Shield, they will not lose the key if they take damage. If the player successfully brings a Special Stage Key to the Goal Ring, they will warp to the Special Stage.
In Super Hard Mode, all Special Stage Keys are replaced with Power Cores.
In Sonic Advance 3, the key is referred to as the Special Key. In order to find a Special Key, the player must find all ten Chao hidden in a Zone. Once that is done, the player will be able to find the Special Key for that Zone in any of its Acts. Upon finding a Special Key, the player must then finish the Act in order to keep the key, then they can use it to unlock the Special Spring in the corresponding Zone's Map to access the Special Stage. Once a Special Key has been used, it disappears, and the player can retry the Special Stage as many times as they have keys remaining, up to a maximum of nine per Zone at a time. If they have no Special Keys left, they will have to replay an Act to find another one. Special Keys cannot be collected or used in co-op play.
In the 2004-03-01 prototype, the player doesn't need to finish an Act to keep a Special Key; the player could just exit the Act via the pause menu and they will keep the key.