This button lets you mark a client who doesn't attend the class.
The client is charged according to your cancellation policy, and "no-show" will appear on this date in their booking history.
When you tap this button, empty checkboxes appear beside each client. For any client that doesn't turn up, tap in the circle beside their name to mark them as no-show.
Here all all the main help categories in the Appilates user manual.