A perpetual fallback license is a license that allows you to use a specific version of software without an active subscription for it.
When purchasing an annual subscription, you will immediately get a perpetual fallback license for versions released within your license term.
For example, if you purchase a yearly subscription on March 10th, 2021, your subscription will run until March 10th, 2022. Any versions of the software released between those dates you can use without renewing your subscription. If your subscription has expired, you will not receive email support.
Monthly subscriptions are not applicable for perpetual fallback licenses.