The All Tasks Closed Controller field is a system field that is enabled when all the tasks linked to a record (incidents, change requests, and problem) are closed. A system field is a field that is used in code and must not be shown on the layout of a form. This field is enabled by code and used in the following workflows:
An email message is sent to the owner of a record when all the tasks linked to a record are closed. Similarly, you can use this field in triggers, workflows, and so on as per your requirements.