The Use item ability will play the use animation for the item (shooting, melee slash, grenade throw, etc). This ability is setup so multiple Use item abilities can be added to a single character depending on what type of use the character should perform. As an example an assault rifle can be fired with the left mouse button, but it can also be used as a blunt object in a melee attack. This is done by adding two different Use abilities to the character and specifying a different Action ID.
The slot that should be used. If a value of -1 is specified then the ability will use all of the equipped items.
The ID of the Item Action component that can be used.