merc crossbow offhand

in one of the promo or gameplay videos it showed a object in the offhand of a crossbows build for one rune slot and stuff but where is it? or what is it called

Just rewatched the Mercenary gameplay overview vid and saw what you mean.

This vid showed a system similar to the quiver for bows called "attachments".

I would love this to be in the game to provide passive buffs for crossbows, as this was a feature I was really looking forward too.