what is this tier "X"

Quite so

 

Wasn't Tier X just a cover term for tiers after 10? Like, using X as a standard variable?

I've seen devs refer to things as tier 10/X

 

Uhhh... Just speculating here... x is a common variable when some doesn't want to type tier 1 tier 2 tier 3 4 5 6 7 8 9 10... I mean. A variable in general. Tier x could be any number from negative infinity to infinity. Since it's been said 10 tiers, it's assumed tier x = tier 1-10, no?