Outside of a modulation attenuator, PS-20's main VCA doesn't any controls, per se, but it's helpful to understand its functionality.
VCA is an acronym for "voltage-controlled amplifier," which is a fancypants way of saying, "volume knob, controlled by voltage." The sideways triangle on the panel is the standard schematic symbol for an amplifier. The horizontal lines on either side indicate the signal flowing through it, and the EG 2 & Ext labels with upward arrow show the incoming voltage controlling the amount of signal flowing through. A simple analogy would be to think of the horizontal line as water flowing from left to right, and the control voltage from the bottom as a faucet that gets adjusted by the incoming control voltage.
The VCA is hardwired for control via EG 2, but it can also be controlled with EG 1 using its output in the patch panel, patched to the VCA Amount jack.
VCA Modulation Attenuator Control
Mod Gen/Patch Panel- Sets the amount of modulation signal routed to the VCA either via the hardwired internal MG saw/triangle/ramp wave or from the patch panel VCA Mod input jack when a source is plugged into it.