From the Studio One Reference Manual: "Auto-Gain: Engage to automatically fix the 0 dB input level to the 0 dB output level (guarantees that a 0 dB input level results in a 0 dB output level)."
The Presonus compressor plugin functioned exactly like this in Studio One 2. Now that I have upgraded to Studio One 4, the auto-gain button is broken and it makes all of my presets useless. All it does now is add around 1dB of gain, regardless of how the other parameters are set.
I will repost some images from this thread (https://answers.presonus.com/12712/doesnt-compressor-auto-gain-studio-work-like-does-studio-one) to illustrate what I am talking about:
Here is what it looks like in Studio One 2:
and in Studio One 3 or 4:
The answer in this thread doesn't make sense. This is not an improvement, it is a mistake -- the feature no longer functions as described.