From Google AI:

The Boss CS-3 Compression Sustainer "Silver Label" is a standard production variant of the pedal manufactured from 1997 through late 2017. It features a sticker on the bottom instead of the later "black label," and houses a high-performance VCA circuit that reliably evens out dynamics and adds sustain.

Core Features

• Level: Controls the overall output volume (makeup gain) to compensate for the compressed signal.
• Tone: Boosts high frequencies when turned clockwise, and cuts them when turned counter-clockwise.
• Attack: Controls the speed at which the compressor reacts to your pick transient; slower settings allow more initial pick punch to come through.
• Sustain: Adjusts the compression ratio and the length of the trailing note sustain.