Task closing rules help administrators mandate task-related fields which they consider as significant for a task. The technician working on the task will have to adhere to the rules, if a task closing rule is configured; else an error message will be displayed prompting them to fill the mandatory fields.
For example: If you set Worklog as mandatory field, the technicians will be able to complete tasks only after filling in the Worklog details.
Go to Admin > Automation > Closure Rules.
Navigate to Task.
Select the task fields you want to mandate for closing a task.
Enable Task Completion Status to consider only the closed status of task as completed status.
Click Save.