Although this article is based on official information, the actual name of the article's subject is pure conjecture.
S is a Monitor power-up in the Sonic the Hedgehog. Unlike many other power-ups that are seen in the earlier Sonic titles for the Sega Mega Drive, the S is not seen through regular gameplay. Because of this, the player is able to find them only after activating Debug Mode. It is also notable that monitors with S on it don't usually work in several games while they are discovered as unused content of the game. Its appearance and abilities varied based on its game appearance.
Sonic the Hedgehog CD
In Sonic the Hedgehog CD, the S's Monitor displayed an orange-colored "S". The player must have enabled Debug Mode to place S Monitors in the level. When broken, it granted the playable character the abilities of both Invincibility and Power Sneakers at once, meaning that his running speed vastly increased and was invulnerable to enemies and obstacles that would hurt him. These effects only lasted for a short amount of time however and will eventually wear off.
Sonic the Hedgehog 3 & Knuckles
The S reappeared later in Sonic the Hedgehog 3. Unlike its appearance in Sonic the Hedgehog CD, the S Monitor's icon was changed to a blue-colored "S". Its abilities also changed; it granted the user his Super State and 50 Rings when broken. Like Sonic the Hedgehog CD though, the player has to activate Debug Mode in order to place it in a level. (Here, the S is basically a Monitor without displaying anything.) In Sonic's case, after he breaks the monitor, he will instantly transform into Super Sonic. In Tails' case though, although he still unofficially transforms into "Super Tails", his sprite will be glitched as Tails is not intended to have a Super State in the game.
In the locked on version of Sonic the Hedgehog 3 & Knuckles, S functions identically although when playing as Knuckles, the "S" is colored red instead of blue. When broken, Sonic and Knuckles transform into Super Sonic and Super Knuckles respectively (in terms of their abilities) but will emit afterimages as if they were in their Hyperforms, while Tails transforms into Super Tails as normal. If Blue Knuckles breaks the item box, he will become yellow with glitched afterimages and cannot breathe underwater.
Sonic the Hedgehog (1991)
While the S did not make an appearance in the original Sonic the Hedgehog, it was apparently dropped during the game's development. Despite this, unused sprites of monitors with S on it were discovered in the files. According the issue of Computer & Video Games, this power-up could have granted both Invincibility and Speed Sneakers at once (which would later be implemented as the S's ability in Sonic the Hedgehog CD).
They made their reappearance in the 2013 re-release of the game as a placeable object in Debug Mode where their appearance is identical to that of Sonic the Hedgehog CD but its abilities are the same from Sonic the Hedgehog 3 & Knuckles.
Sonic the Hedgehog 2
The S is featured again only in the 2013 re-release of Sonic the Hedgehog 2 where its appearance and abilities are identical to that of the 2013 re-release of the original Sonic the Hedgehog. This is the only method for Tails to achieve Super Tails in this game.
Due to Sonic and Knuckles sharing different color palettes in Sonic the Hedgehog 3 & Knuckles, the S monitor is colored red instead of blue, when the playable character is Knuckles. The Water Shield shares this trait.
If you use one and then quickly turn back into item mode when you finish the letter, the name of the character you're using and/or the post will turn into the colours of the hyper form.
In Sonic the Hedgehog 3 & Knuckles, should the player destroy multiple S Monitors at once, the player's controls may suddenly be locked. If the player then presses Start, he/she will be redirected to the game of Sonic & Knuckles and will play through the levels there.
This is the only way to enable the use of S Monitors (let alone Debug Mode) in the said game.
In Sonic the Hedgehog 3 & Knuckles, each time the player breaks an S Monitor the user will perform their transformation animation even if he was already in his super form. In the 2013 re-releases of Sonic the Hedgehog and Sonic the Hedgehog 2, the S Monitor simply granted 50 Rings without the user performing his transformation animation if he is already in his Super form.
In the 2013 re-release of the original Sonic the Hedgehog, there is a glitch that the S Monitor may not grant its abilities when broken.