Why there is a + after OFF? Isn't just OFF enough?

My best guess is that OFF is not enough, but to add a "+" to denotes another category.

 

It is just like in the examination in the school when we were young, an A is not enough, but there is A+, A-.

 

 

 

Can it be Offense Extreme?

Never heard of Offense Extreme, but the norm is that it follows the following guide :
DEF-, DEF, DEF+, All Round, OFF-, OFF, OFF+. (This is just a rough guide from Butterfly, other manufacturer may have different guides)

Joola has an Offensive Extreme category.

Never heard of Joola's Offensive Extreme category. I think Xiom has their own category as well.

This could be that individual manufacturer has their own marketing terms to promote their products and none can be campared theoratically except being used to play and compare the result.