Updates to the OTB software are released periodically (perhaps 1 or 2 times per year) and include bugfixes and enhancements. Updates are available to all schools whether or not they have a current Annual Support Agreement. They are applied automatically if you have an ASA.
Updates to (online) OTB documentation are also available to all customers.
Updates to the underlying Operating System are required less frequently, perhaps every 5 or 6 years. These are applied free of charge for users with a current Annual Support Agreement.