The AGC will set the overall gain profile depending on you AGC spec point. It will reduce gain when audio is present and then allow levels to creep up in extended quiet periods but not all the way to your chosen AGC spec point. The AGC spec point determines the average peak level the AGC will try to achieve for a moderately large input level (the input gain is determined by the Audio Input Reference Level for Ch1/2).

 

A higher Spec Point level (smaller number, eg -6db) increases the target peak level, so provides a wider recording range. During very sudden input peaks if the level reaches your chosen AGC spec point very aggressive gain reduction occurs, compressing the entire audio signal leading to undesirable pumping effects or audio distortion but typically avoiding hard clipping.

Using a higher Spec point reduces the chance of normal audio peaks triggering this very aggressive gain reduction due to the larger available range, but if you do hit the spec point the gain reduction is more aggressive and the resulting audio may sound even more distorted.

The Limiter:

The limiter which only functions in manual mode kicks in when the audio hits the level you have set. The aggressiveness of the limiter depends on the set level. When using a lower level (larger number eg -17db) there is more range between the onset of compression and the clip point so it can be less aggressive but more of your normal lower peaks will be compressed (depending on the gain you are using). The limiter only acts on the louder part of the input rather than the overall gain so pumping etc is much less obvious than when using AGC.