When someone books, they get a confirmation email from your business: the date, the price, any deposit, directions, a link to manage the booking, your cancellation policy and a calendar file that adds it to Apple, Google or Outlook in one tap.
Reminder emails go before the appointment, on every plan. Under Shop settings, Booking rules, Reminders, Reminder emails is No reminders, Day before (the default), Day before and 2 hours before, or Two days and day before. A booking that has been cancelled is never reminded.
Reminder texts are on Pro: 50 texts a month for each person on your plan. Reminder texts sits beside the emails with the same times. They start off, because texts cost money, and a text goes only to a customer who gave a phone number. The email reminder still goes at its own times. A text that runs to two parts counts as two, and the month's allowance starts again on the 1st. When texts run low, a bar tells you on the dashboard, the diary and Messages, and you can buy more under Account & plan, Your plan: 100 for 5 GBP, 500 for 22.50 or 1,000 for 40. Bought texts never expire and are used after the month's included ones.
When you add a booking in the diary yourself, Send a confirmation email is ticked and Send a confirmation text is not; a confirmation text comes out of the same texts.
To say it in your own words, Emails under Shop settings lets the confirmation, the reminder and the thank-you each take your own subject and message, with a preview beside them.