The Engagements feature helps you move toward a more structured "productization" of your coaching offering (such as a 10-session package, 3 months with a weekly call, or simply a monthly membership fee).
Use Engagements to keep tabs on how much time or how many sessions are left by looping in with appointment scheduling. You can also set up matching invoicing and any notifications to both yourself and the client (like "You've got 2 weeks left in your package; want to renew?")
Engagements are also the way to set up automatic recurring billing (for example, a subscription model), even if no appointments are included.
First things first: make sure Engagements is enabled on your account. There's no extra charge for using Engagements; the option to enable or disable it exists because if you know you DON'T want to use it, you can have a slightly more streamlined system by turning it off.
To make sure it's enabled for your account, click on your name in the upper right >> My Account >> Add Ons.
When enabled, you'll find the place to set things up in Settings >> Engagements.
This video will guide you through setup.