It sets how the renewal date is derived when a subscription is created or renewed:

• Renew on current renewal date for the last week available (default) - the renewal weekday is the one chosen at purchase, and admins get an Update next renew day to field in the pause / unpause / cancel-day pop-ups on the subscription page.
• Renew on last day of deliveries - the renewal weekday is automatically set to the weekday of the plan's last delivery when the plan is saved, and the manual Update next renew day to field is hidden from the admin pop-ups because the date is derived from the delivery calendar.