What's the meaning behind the rarity item's classified in Anime/manga? In anime, i always seen some item rarity that classified like 


S (super)
SS (super super (?) ) maybe... i dont know


and many alphabetic words. Why did they add like "S" for super on the classified of rarity item like that ? Why they dont use like "A+","A-","B+" just like score result of an exam ? What is the story behind it ? What's the first anime/literature that used this kind of method ?

Ex. Classified Greed island Card Rarity in Hunter x Hunter
It not generally used in reallife. It is originally come from Japanese Animation &amp; Games, is generally come from games. In real life school ranking mark only A, B, C is credit marks, this will feel not enough for high ranking. In Games, is freely to input A+, A-, B+, B- rank system, but games generally put on SSS, SS, S, A, B, C, D, E, F, make players feel more happy on their result.

It means Super or Superior or alike surprise. Superlative classification. Super x3, Super x2, is weird to put Sx2, Sx3, so it is SSS, SS.

Some of games you can see R, SR, SSR, UR. R is meaning "Rare", mostly in card games. That means you get a rare card. SR is Super Rare. SSR is Specially Super Rare sometimes. UR is Ultimate Rare.

Rare[R]Super Rare[SR] Card Sample , Ultimate Rare[UR] Card sample

Edit

I remember I learned about this game s-ranking system, but I don't remember who is the author of making this system, sorry. I remember have origin author who make this s-rank, but not remember the name, this is minor title in history. 
Such as statement "The atom bomb S-Ranked Hiroshima" from Urban Dictionary: S-Rank. And, SS-Rank is by The early rank system of 1926, insignia of the Schutzstaffel. Early military.

Information is here:
   Giant Bomb: S-Rank,
S-Rank_Japanese