AMS (Annual Maintenance Subscription - http://www.pdftron.com/support/maintenance.html) is optional maintenance and support subscription issued on a per-license basis with each product license.
Owners of an AMS are entitled to all minor and major product version releases. Typically, PDFTron releases anywhere from 1 to 2 major product releases as well as a number of minor updates each year. AMS also includes priority tech support.
For more info and steps to renew AMS please see the above page.