If you add a customer to a schedule and if its outstanding invoices are in another schedule, you will see an ‘?’ icon next to it. The same happens if the customer currently has no outstanding invoices.
Having the mouse pointer over the icon also explains this.
The calendar icon informs you in which schedule an invoice or customer is at the moment.
In the example below, we are in the schedule Good Customers. The customer Filipenterprises is in the schedule Tardy Customers, whereas its invoice INV-4321 is in the current schedule.