What is the difference between fan 'on' vs 'circulate'?

Hi – I believe that this is a function of the thermostat and not particular to the furnace. My t-stat is one of the Honeywell VisionPRO series and the operating manual says:
● On: Fan is always on.
● Auto: Fan runs only when the heating or cooling system is on.
● Circ: Fan runs randomly, about 35% of the time. Use this setting for improved air circulation, or for more efficient air cleaning, if you do not want the fan to run all the time.