"Circulate" is a feature that puts the device in an automatic state. It allows you to set a schedule that automatically turns ON and OFF for the desired amount of time.
Example: You can set the Circulate timer for an eight-hour period and set the plug start duration for 2 hours and the end duration for 4 hours. This feature would turn the plug on for 2 hrs, then off for 4 hrs, then back on again for 2 hrs.