The Red 3 from Focusrite is a Class A, VCA-based dual mono/stereo compressor/limiter. The compressor/limiter features a single Class A VCA to achieve truly high class compression and limiting, free of the usual compromises.
A key input button switches control of the compressor to a signal entering the unit through the key input. By attenuating any problem frequencies in a second feed of a mix and sending that to the compressor's key input, the original signal is compressed as if the dominant instrument had been attenuated. This technique can be useful when compressing bass-heavy dance music.
In order to separate compression and limiting, the side-chain electronics contain three VCAs in series to generate compression and limiting control voltages which drive the main VCA. The result is true compression followed by limiting, rather than the more common characteristic of compression that turns into limiting.