An Adventure Backpack cannot be inserted inside another. Access the Backpack's GUI to mount it, then use the default key bind B to access it.
Whenever a backpack is dropped, it keeps all the items stored inside, it may also be placed rendering it immune to the effects of lava. The 2 tanks (one on each side) hold 4,000 mB of liquids each and can be manually filled with a bucket or using an Hose from which the Player can suck or spill liquids and even drink for some interesting effects. The backpack can hold 2 tools, which can easily be swapped with a tool in the hotbar ( Shift+Scrolling), the tools show on the backpack's model. It functions as a portable chest, sleeping bag, crafting table and tank and also features a quick swap tool holder. The Adventure Backpack (Standard) is an item added by Adventure Backpack mod.