COMPRESSOR SECTION

Threshold: -20dB to about +10dB.
Ratio:1.5:1 to 6:1.
Release:100ms to 1.5 seconds.
Slow Attack:Standard setting is fastest and is designed to catch transient spikes. When pushed in, the attack response time slows down by a factor of about 10 to compensate for slower moving parts like vocals.
Key In: When pushed in it switches the detector's sample signal from the standard input signal to the "key input" jack on the back of the unit.
Compress In: Engages the compressor.

LIMITER SECTION

Threshold: +4 to +15dB.  This is post makeup gain, so raising the makeup gain can raise the level of the output signal enough trigger the limiter.
Release: 50ms to 600ms.
Fast Attack: Cuts the attack response time for the limiter in half.
Limit In- Engages the limiter section.  The limiter is triggered from the output, so the compressor section's signal reduction will reduce the signal's triggering of the limiter.  The limiter could also work without the compressor engaged just as the compressor can still work without the limiter engaged.
Stereo Link: Puts the unit into stereo mode.  This runs both channels of compression and limiting from the detector in Channel 1.  In link mode, Channel 1's compressor and limiter settings control both channels.  Input trim and makeup gain settings for each channel remain independent of each other.
Makeup Gain: Compensates for the signal reduction from compression.  Bypassing the compressor bypasses the makeup gain stage.
Gain Reduction Meters: Monitors the amount of signal reduction by both the compression stage and the limiter stage.  In stereo mode, both meters will track the same because both are triggering from Channel 1's detector circuit.