If you have a subscription plan with Terms, the renewal reminder email is set in advance of the last renewal, allowing the user to opt into another set of renewals.
One way to think about Subscription Terms is that it is a contractual obligation. If the customer subscribes for a monthly subscription, billed monthly, they do not have the option to cancel mid-term, so the Renewal Reminder email is not sent - they are considered a forced renewal.
Instead, this same customer will receive a renewal reminder email in advance of their 11th monthly cycle. At that point, canceling will result in the subscription ending at their 12th month, or they can renew for another 12 months of monthly billing