Just going to leave this here:

Imagine X rank weaps, let's say their attack has a value of 1-100 with 100 being best tier and 1 being lowest tier.
Let's say now there is X+, the difference might be 200-1000. 1000 being best tier, 200 being lowest tier.
Then later there is X++, the difference might be 1000-20000. 20000 being best tier, 1000 being lowest tier.
Those who manage to get their hands on high tier weapon have higher growth potential than free players, who can only afford lower tier ears.
This is not noticeable at S+ yet, but in the future this difference will get really big.

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I believe the term is progression.

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