Hello everyone,
I think I can shed some light on this for once.
BIS stands in this context as a gaming acronym for "Best in Slot".
It refers to a piece of equipment that is the best piece you can equip yourself with in a particular slot in a game. It is used as a gaming acronym in various games, in games like war or shooter games it can be a weapon or another piece of equipment, it can be armor, here it refers to the clothing items described in this way.
Stay safe and be well you all,
divingA
He who does not submit to the laws, must leave the area where they apply. (Johann Wolfgang von Goethe)