Availability
Many computing sites exclude planned downtime from availability calculations, assuming, correctly or incorrectly, that planned downtime has little or no impact upon the computing user community. By excluding planned downtime, many systems can claim to have phenomenally high availability, which might give the illusion of continuous availability.
Systems that exhibit truly continuous availability are comparatively rare and higher priced, and they have carefully implemented specialty designs that eliminate any single point of failure and allow online hardware, network, operating system, middleware, and application upgrades, patches, and replacements.