HDMI inputs have additional processing to automatically adjust the volume level - particularly useful when the source is broadcast television such as sport and the levels vary considerably. A target level can be set to bring low audio levels up.
The Automatic Volume Control can be thought of as a compressor with attack speeds dependent on whether it is bring the signal up or down. Target level sets the desired level of the input HDMI signal after the automatic gain has been applied. A maximum target of -3dB is possible, relative to the clip point of the Zone unit.
The maximum gain that the control can automatically apply is 15dB. The Volume Ramp Rate is the time in which the gain will climb to the target level. If a louder signal is then present on the input, the control will very quickly reduce the applied gain. Choose a time appropriate to the source material to achieve your target level.
Short Ramp Rates will cause the control to "pump" which will adversely affect the audio - especially with music. Signals below -36dB are considered "background noise" that is not suitable for amplification and are unaffected by the automatic control.