The idea of a voltage-controlled amplifier (VCA) is that an audio or control signal is patched to its input, and externally controlled voltage controlled via the Control Inputs jacks. This is useful for turning audio or control signals on or off, applying envelope volume curves to sounds, regulating the amount of modulation signals applied to audio signals, and more. A simple analogy would be to think of a water faucet, with the control voltage affecting how much the faucet is opened.
Inputs, Outputs, and Controls
Linear (Lin.)/Exponential (Exp.)- Selects the "curve" of the amplifier's response as the input CV rises from 0 to 5V. Lin. or linear response curve is equally proportional across the voltage input range, whereas Exp. or exponential curve is closer to how the human ear perceives volume. You'll likely want to use the Lin. setting for modulation or control voltage situations, and use the Exp. setting when an envelope generator is used to control an audio signal with the amplifier. Or just use whatever sounds best, we won't tell.
Fixed Control voltage- Adds up to 5V of gain. This works in addition to incoming and Control Inputs jack voltages. It's useful for manually "opening" the amplifier for drones.
Control Inputs jacks- Control signal inputs are patched in here. These have no attenuators and are "wide open" all the time - typically these would be used to patch an envelope generator. All three jacks are the same, and when more than one is used, incoming voltages are summed.
Signal Inputs jacks- Use this jack to patch in audio or control signals to be controlled by the VCA. Both inputs are summed.
Signal Outputs jacks- These carry the CV-modified version of input signals.