Usage based subscriptions allow you to give a limited number of class or appointment credits per billing cycle. For example, you could have a subscription that gives 10 class credits per month. This is a helpful approach if you are looking to offer to more restricted (and more affordable) monthly membership than, say, an unlimited one.
Then scroll to 'Usage limit settings' > toggle on 'This subscription has a usage limit' > choose how many class credits to give & how many appointment credits to give.
Note that you can combine these (allow them to be used on either classes or appointments by using the toggle for 'Combine usage limit between classes and appointments.'
The last two settings to be aware of are:
'Separate usage limit from renewals' which allows you to give credits more often than the renewal occurs. (ex. give 3 credits a week, while billing monthly for 12 credits). This helps customers that have a tendency to use all their credits up in the first week and have to wait until month end to be restocked.
Auto-renewal settings: toggle this on if you want your subscription to automatically renew (charge) every month (or whichever billing cycle you chose above
And, as always, make sure the subscription includes the classes and appointment services you want it to be used on.