Tier list ranks
The S, A, B, C, D and F rank labels used to sort items in a tier list from best to worst.
The scale is a convention, not a rule. Most tier lists run S down to D and add F when they want a rank that reads as a failure, but you can rename or recolour every row and use as many as you need.
S tier
S tier is the highest rank on a tier list, placed above A tier and reserved for the small number of items considered clearly better than everything else.
A tier
A tier is the second rank on a tier list, sitting directly below S tier and holding strong options that fall just short of the very best.
B tier
B tier is the middle rank on a tier list, used for solid, viable options that are neither standouts nor weaknesses.
C tier
C tier is a below-average rank on a tier list, used for options that are workable but noticeably weaker than the tiers above them.
D tier
D tier is a low rank on a tier list, used for options that are clearly outclassed and rarely worth choosing over anything above them.
F tier
F tier is the lowest rank on a tier list, borrowed from the failing grade in the American school system and used for options considered outright bad.
Why S sits above A
The ordering looks wrong to anyone expecting an alphabetical scale, and the reason is that two different grading systems were spliced together. A through D comes from American school grading. S comes from Japanese grading, where a mark above A was used for work that was exceptional rather than merely excellent. Japanese video games adopted S for performance grades, western fighting game communities picked the convention up when they started publishing character rankings, and the hybrid scale stuck.
Why there is no E tier
The same borrowing explains the gap. American school grades run A, B, C, D and then F for fail, skipping E so that it could not be misread as a passing mark or as "excellent". Tier lists inherited the whole sequence, gap included, which is why a scale that otherwise looks alphabetical jumps straight from D to F.
Common variations
The six-rank scale is a default, not a standard. When S alone is not emphatic enough, people add SS, S+ or EX above it - again following Japanese games that escalated the same way. Lists that want to stay positive stop at C or D. Lists ranking something with no bad options sometimes drop letters entirely and use named tiers instead, which works just as well as long as the order is obvious to a reader who has not seen the list before.
How many tiers you should use comes down to how fine a distinction you can actually defend. Four or five ranks suit most subjects. Past seven, placements start to feel arbitrary, and the tier list has quietly turned back into the strictly ordered ranking it was meant to replace.