Advice needed: Suspension settings

[Ball and Chain]

My rear shock has 2 knobs for setting the suspension.  The prominant knob sets the hard/softness.  It's not obvious what the other one sets.

What is the ideal for tar and for gravel?

 

[lecap]

 

[Ball and Chain]

 

[lecap]

 

[shark_za]

 

[Ratel]

 

[YellowFever]

 

[Ball and Chain]