A perpetual license includes one year of maintenance when you buy it. Renewing is optional. The registered user is notified by e-mail 30 days before maintenance expires, and again on the day it expires.
An annual license includes maintenance for as long as the license lasts, and both end together. Renew the license to continue using the software.
From the Administrator's Guide for Analyse-it 6.24.0: https://analyse-it.com/docs/user-guide/maintenance