1, 2, 2.25, and 4 Gallon Extension :

Use in conjunction with a pack mount (RX-PM, RX-PM-DLX, RX-PM-LOX).

1.75 Gallon Extension: 

Use this Extension to stack any Fuel Pack on top of the 1.75 Gallon. Use the standard Extension if stacking the 1.75 Gallon on top of a 1,2 or 4 Gallon Fuel Pack. 

3 Gallon Extension:

Use this Extension to stack any Fuel Pack on top of the 3 Gallon RotopaX, 3.5 FuelpaX, or 4.5 FuelpaX. Use the standard Extension if stacking the 3 Gallon on top of a 1,2 or 4 Gallon Fuel Pack.