timeSensor LEGAL marks the invoices due for reminder with a grey dot under Finances - Debtors - Open invoices. This article describes how timeSensor LEGAL calculates whether a grey dot should be shown or not.
The starting point for the calculation of further reminders is the date of the last reminder. If the invoice is not yet due for the next reminder, you can edit the reminder status. For example, you can change the date of the previous reminder or delete the last reminder.
Added to the invoice date (or date of the last reminder) are the settings of the current dunning level of the dunning scheme assigned to the invoice:
Finally, a tolerance is added. This is five days by default.

If you want to change the tolerance, please contact the hotline, as this parameter is not implemented on the GUI.
Thus we can answer the initial question: timeSensor LEGAL sets the grey dot if the invoice date (or date of the last reminder) plus the payment deadline according to the current reminder level of the assigned reminder scheme plus tolerance is smaller than the current date.