I noticed if I put a item in my inventory into the backpack on another gear item it puts it in the inventory slot that takes that item; however, when I put the item over a none gear item it is lost. It could be put in another slot and if backpack is full could have a stop message. Also, I am not certain what happens to gear if item from inventory is placed over an item that is already filled in the inventory. It could be moved to an open slot in the backpack or get a stop message to move or remove items.
