See Capture Techniques - Quad Cortex Wiki. Basically, -12 dB for clean captures and -6dB for distortion captures. These are just conventions so that the captures have similar levels. -12dB for cleans ensures there’s headroom to avoid output limiting or digital clipping for clean spikes. -6dB for distortion is just a consistent, reasonable level that seems to somewhat match what NDSP used for the amp blocks.