If a customer does not show up for an appointment, you can store this info in the agenda and in the customer profile. 

To register a no-show, click on an appointment in the agenda and then on the right-hand side the “edit” button. Next, on the right, check "no-show" and save the changes.

In the agenda there is immediately a button on the right: no-show appointment

You can recognize a no-show in the agenda by the "cross" in the right corner of the appointment, and there is also a message in the customer profile.

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In the tab "appointments" no-show appointments are highlighted in red so you know which appointment it was to which someone did not show up.
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No-shows can also be used in the customer filter, if you want to see a list of customers where a no-show applies (and how often).