Backpacks are an item from Forestry that can be used for expanding your inventory for specific types of items. The basic form of these have 15 slots, but they can be upgraded to hold 45 stacks of items. Backpacks themselves do not stack. These are crafted on the crafting table; the upgraded versions require use of a Carpenter.
The Miner's Backpack holds ores such as iron, copper, tin, diamond, coal, silver, etc. (Full listing on the Miner's Backpack page)
The Diggers Backpack holds things such as cobble, gravel, dirt, flint, and clay.
The Foresters Backpack holds wood, saplings, sticky resin, leaves, seeds, wheat, flowers, and mushrooms.
The Hunter's Backpack holds anything that drops from a mob; from eggs and feathers, to ghast tears and blaze rods.
The Builder's Backpack holds materials that could be used to build with: brick, stairs, fences, glass, smooth stone and torches.
Adventurer's Backpack Edit
The Adventurer's Backpack holds nothing by default. To change this you must go into the Forestry config and change the item id's allowed to be placed in the backpack.
The Thaumaturge's Backpack holds zombie brains, elemental shards, quicksilver, thaumium, golems, Essentia jars and knowledge fragments. (Need more input here). (Does not work in latest FTB Infinity, cannot carry anything - Feb 6th, 2016)
The Trackman's Backpack holds items essential for railway tracks, holding every type of track. It also holds the Crowbar and the Trackman's Goggles.
The Iceman's Backpack is capable of holding Snowballs, Snow Blocks, Ice, and Ice/Snow Walls. But it also has the ability to compress Snowballs into Snow Blocks or Snow Blocks into Snowballs if desired. It does this by attempting to keep no more than 15 and no less than 8 Snowballs in the bag at any time.
Stores any ComputerCraft and MiscPeripherals block or item.
Woven Backpack Upgrade Edit
All backpacks can be upgraded to go to 45 slots in the carpenter as follows:
You can toggle the mode of the backpack's operation by holding Shift (to crouch) and Right Clicking with the backpack in-hand.
Normal - This mode takes the items allowed in the backpack from the ground and puts it directly into the backpack saving space in your inventory
Lock - Can place items into it manually but does not pick up items
Resupply - If it is holding the materials it refills the stack of material you are holding as you lose some of it.For example, when building a house, you place one brick block down and the stack will be refilled back to 64 after placing the brick (assuming the backpack has the material).
Receiving - When crouching,right clicking on a chest will take all matching items from the chest
Additional functionality Edit
When not in receiving mode, you can crouch and right click on a chest with a backpack to dump the contents of the backpack into the chest. This will not change the mode.