This excerpt taken from the ARAY 10-K filed Sep 4, 2007.
Leverage our installed base to generate additional recurring revenue. We have designed the CyberKnife system so that customers may upgrade their previously purchased systems as we introduce new features. We generate additional revenue by selling multiyear service plans that provide eligibility to receive upgrades, when and if available. These contracts are typically signed at the time of CyberKnife system purchase and generate additional revenue throughout the life of the contract. In addition, we sell upgrades to our existing customers who are not covered by service plans or who have exhausted the upgrades deliverable pursuant to their service plans. Finally, we offer shared ownership programs, which enable customers to reduce the upfront investment required for the CyberKnife system in exchange for sharing a significant portion of revenue with us that is derived from each procedure.