The Active Indicator option places icons in the viewports that indicate which datasets are currently active in the Display.
To turn on this option, go to the View menu and select Active Indicator.
The Active Indicators appear in the upper right corner of the viewports.
The top, middle and bottom indicators correspond to the Reference, Input 1 and Input 2, respectively. If the indicator is a solid circle, the corresponding input is on. If the indicator is a hollow circle, the corresponding input is off.