The size of the gas container can be controlled by pressure or by volume:
When the volume slider is in control, it sets the size of the container - while the pressure of the gas (shown by the pressure slider and the readout below) varies freely.
When the pressure slider is in control, it sets the pressure exerted by the piston on the gas below - while the volume varies freely as the piston moves up or down. The pressure of the gas (shown on the readout in N/m2) changes as the piston moves to its new position.
what is pressure?