There are four basic maintenance activities. Corrective maintenance: It includes correcting an error discovered during operations. Adaptive maintenance: It includes activities to arrange exiting functions or adding new functions such that the system conforms with job or activity changes. Perfective maintenance: It means improving and modifying the system to respond to changing user requirements and organizational needs, enhancing documentation and increasing system efficiency, reliability, or maintainability. Preventive maintenance: It is related to lengthening the service duration of the existing system, to reduce the likelihood of future system failures. Although this type of maintenance has the lowest priority, it adds high value to the organization.