The Tank is a smaller version of the Backpack's internal Oxygen storage. It can be attached to any slot on the backpack, including its widget slots.
When connected via a Tether to a base module or vehicle, the Tank will fill with Oxygen. The Tank only begins filling once the main backpack tank is full.
The Tank attaches to any 1-slot and stores as much Oxygen as the backpack's main tank (70 seconds when idling, 50 seconds when running); having one Tank in the backpack doubles the player's Oxygen storage.
When untethered, the Tank's Oxygen is consumed prior to the backpack's. The oxygen gauge on the tank visibly empties as its capacity runs dry.