This icon
indicates an Overnight, and it appears whenever the system sees an Overnight condition.
OPTIMAL counts an Overnight condition occurring whenever any visit passes through midnight.
For example, if your time starts on Tuesday at 4:00pm and ends on Wednesday at 9:00am, this is considered an Overnight event. Similarly, if your time starts on Tuesday at 11:00pm and ends on Wednesday at 9:00am, this is also considered an Overnight event.