0dB is 100% gain - turning down the volume attenuates the gain, e.g. -10dB etc.

Overdrive is the area above 0dB - e.g. +5dB - in which the gain is above maximum. This allows quiet tracks to be made louder, but if a track already uses the full dynamic range distortion will occur.

I'll put the userguide online after it has been updated. It hasn't been updated yet because the UI for a few of the new features hasn't been finalised, but we're getting there.

Rob