Once a system is deployed it is in service for years and decades. During this time the system and its operational environment is often corrected, changed or extended.

What is Maintainance Testing ?

When any changes are made as part of maintenance, maintenance testing should be performed, both to evaluate the success with which the changes were made and to check for possible side-effects (e.g., regressions) in parts of the system that remain unchanged (which is usually most of the system).