Although this article is based on official information, the actual name of the article's subject is pure conjecture.
S is a recurring Monitor power-up in the Sonic the Hedgehog series. Unlike many other power-ups, the S is not seen through regular gameplay. Because of this, the player is able to find them only after activating Debug Mode. Its appearance and abilities varies on its game appearance.
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 grants Sonic the abilities of both Invincible and Power Sneakers at once, being able to run at extreme speeds and becoming invulnerable to enemies and obstacles. These effects only last for a short amount of time (for about 15 to 20 seconds), and eventually wear off. There is one hidden underneath the first boss at which can only be accessed using Debug Mode. The Power Sneakers music will play upon opening.
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 have 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 represented by a blank Monitor.) 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.
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 to an issue of Computer & Video Games, this power-up would have granted both Invincible 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.
The S returns in Sonic Mania, and its expansion Sonic Mania Plus, with a similar appearance and the same behavior as in Sonic 3 and Knuckles. It doesn't appear normally in stages, and can only be placed while in Debug Mode.
From version 1.03, the S logo is present in the top-right corner of the screen whenever the playable character is ready to transform into their Super State (having all seven Chaos Emeralds and at least 50 Rings).
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 and the Invincibility sparkles share this trait.
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. This no longer happens in the 2013 re-releases of Sonic the Hedgehog and Sonic the Hedgehog 2 as well as Sonic Mania.
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, they 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 said game.
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.