Similar to defining a task failure, it is also possible to define what constitutes a changed task result. This capability is particularly useful with the command family of modules (command, shell, raw, and script). Unlike most other modules, the modules of this family do not have an inherent idea of what a change may be. In fact, unless otherwise directed, these modules only result in failed, changed, or skipped. There is simply no way for these modules to assume a changed condition versus unchanged.
The changed_when condition allows a playbook author to instruct a module on how to interpret a change. Just like failed_when, changed_when performs a test to generate a Boolean result. Frequently, the tasks used with changed_when are commands that will exit nonzero to indicate that no work is needed to be done, so, often, authors will combine changed_when and failed_when...
