Release updates are cumulative, version-specific updates, built to resolve a customer-specific issue on a production deployment.  In general terms, release updates should only be applied to customer environments that are experiencing specific behaviors addressed by a given patch version.  Release updates are available for all customers with an active support agreement and may be requested by completing the request form linked here.  Square 9's fulfillment team will always deliver the most current release update version.  Again, all release updates are cumulative.