In a PCP air rifle, the plenum is the reservoir that stores the regulated air of the rifle.
This is a space between the valve and the regulator.

This 63cc SHP plenum is a major upgrade for the FX DRS 500 and 600 with the standard HP plenum!
Due to the larger volume of regulated air, the DRS gains greater efficiency and power potential.

So if you plan on using heavier projectiles or shooting projectiles at higher velocities, you need the high capacity SHP plenum.

The SHP plenum also has a mounting point for an Angled Manometer Bracket (not included).
This allows you to actively monitor the regulator pressure while shooting!