UAD Powered Plug-Ins Manual v5.2 - Just Music

When Ratio is changed, the Threshold value is updated accordingly:
When Ratio is set to 2:1, the Threshold range is -55dB to 0dB.
When Ratio is set to 4:1, the Threshold range is -45dB to +10dB.
When Ratio is set to 10:1, the Threshold range is -40dB to +15dB.
Note: When Ratio is changed, Threshold numerical values are updated but
the Threshold knob position does not move.
As the Threshold control is decreased and more compression occurs, output
level is typically reduced. Adjust the Gain control to modify the output to compensate
if desired.
Ratio
Ratio determines the amount of gain reduction for the compressor.
For example, a 2:1 ratio reduces the signal above the
threshold by half, with an input signal of 20dB being reduced
to 10dB.
The available Ratio values are 2:1 (default), 4:1, and 10:1.
Attack
Attack sets the amount of time that must elapse once the input
signal reaches the Threshold level before compression is applied.
The faster the Attack, the more rapidly compression is
applied to signals above the threshold.
The Attack range is from 0.10 milliseconds to 32 milliseconds.
The availability of relatively slow attack times (as compared to other compressors)
is one factor that can provide the in-your-face-pumping quality that is so
popular with large console VCA-style compressors.
Release
Release sets the amount of time it takes for compression to
cease once the input signal drops below the threshold level.
The available range is from 0.10 seconds to 1.20 seconds,
with Automatic release available at the full-clockwise position.
The Auto release characteristic for Precision Buss Compressor has a unique
quality that is optimized for program material.
UAD Powered Plug-Ins Manual - 153 - Chapter 16: Precision Buss Compressor