Choose the default category that is selected when someone creates an appointment. By choosing the most popular category you can reduce potential for error if this field is overlooked.

Please note: this field is only visible if Categories are activated in Settings => General Settings.

I've created a few categories by navigating to Settings => Categories. Then I'll select a Default Category in Settings => Appointment Settings. Now that a default is selected when I create a new appointment I can see the default category is already chosen for me and I can change it from the default when necessary.