You can use Message Sequences in a ton of different situations. Do you have a customer that only wants to get reminded once, but you remind all your other customers twice? You can setup a Message Sequence with only one reminder that you can use with that customer.
Do you want to send one template for the first reminder, but then you want a different message to be sent for the second reminder? You can also do that with Message Sequences.
Message Sequences can also help you send reminders to your clients at different times based on the type of appointment or the type of reminder that you're sending them. This can be helpful for early appointments when it might not be very nice to send a reminder 2 hours before an early appointment, as you could for someone with a 1 PM appointment.
Once you create a Message Sequence, you will be able to select it in the Messages dropdown on the appointment creation form.
To create a Message Sequence, you'll want to do the following:
Go to Settings > Reminders.
Click the orange Create New button in the Message Sequences section.
In the Sequence Name text field, enter the desired name for your Message Sequence.
For the First Reminder in the sequence, select the number of days or hours in the "Send immediately  hour(s)/day(s) before the appointment" section to determine when the reminder is sent. (You can also select Immediately if you wish, so the first reminder is sent as soon as an appointment is created.)
In the Message drop down menu for the first reminder, select the reminder template that will be used for your first reminder message.
In the "Send second reminder?" section, click on the Yes radio button to create a second reminder, if you wish.
Repeat Steps 4 and 5 to set up the Second Reminder.
If you wish to set up a Third Reminder for your Message Sequence, select Yes for the radio button and then repeat Steps 4 and 5 again.
Click the green Save button to keep the changes, once the Message Sequence has been set up as desired.
If you decide not to use a Message Sequence after you have created one, and you'd like to delete it, click here for more details.