MusicTechHelpGuy breaks down input gain, output gain, and makeup gain inside Logic Pro's compressor, and why they matter once you start pushing threshold and ratio harder.
Input gain boosts a weak signal before it hits the compressor, so you're not stuck cranking the threshold to compensate for a low recording.
Heavier compression lowers peak volume, sometimes by several dB. Makeup gain restores that lost level after the compression stage, which matters most on vocals that can get buried once compressed.
He skips Logic's auto gain option, finding it usually overshoots. Dialing in makeup gain manually gives tighter control, and pairing a modest auto gain setting with manual pullback is another way to land the level precisely.