Back pack

The back pack [sic] is a miscellaneous item in Fallout and Fallout Tactics.

Fallout
It can be used to organize the player's inventory. A backpack can contain any items that total less than about 40 lbs, but cannot contain larger items like armor.

To use the backpack, right-click to get the command cursor, then left-click and hold on the backpack. Move down to the open hand icon and release to open the backpack. You'll know the backpack is open because its icon replaces yours. Drag items from either hand into the backpack, then click the icon (where your character usually is) to return to your inventory.

Alternately, simply drag the item over the back pack; or if in the backpack, drag the item to the icon (where your character usually is) to move it back to the main inventory.

Fallout Tactics
A single back pack can be found in the inventory of Strom during the training missions. It cannot carry anything, and putting it in your armor slot causes your currently equipped armor to disappear.