S-tier - What does S-tier mean?

S-tier is slang for "Top rank." See an example of how people use it.
An item that is S-tier is among the best of the best in its class. This label is thought to have originated from Japan's school grading system, but it was popularized by Japanese-developed video games.

Why is s-tier often higher than a-tier? - Quora

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What does 's tier' mean on and what is a tier list?