What is the difference between an add-in and an add-on?

Add-in treatments don’t add any extra time to your service. So, if you have a 50 minute massage with an add-in scalp massage, your whole appointment will be 50 minutes. However, if you get a 50 minute massage with an add-on scalp massage, it adds 25 minutes to your total service time – so your service time would be 75 minutes.