These are used to generate SMS text for appointment reminders - both those sent out by the system by the Appointment Reminder Sender, or when the Reminder button on the Workflow|Scheduling screen is used to send an ad-hoc reminder.

When sending SMS Appointment Reminders for appointments in a given schedule, the template used is determined by looking at the location set for that schedule (via Administration|Organisation|Schedule) and then using the SMS Appointment Reminder template set for that location (via Administration|Organisation|Practice Location).

This is the template selection screen for Appointment Reminder SMS Templates.