Sub-task are useful for splitting up a parent issue into a number of smaller tasks that can be assigned and tracked separately. This can provide a better picture of the progress on the issue, and allows each person involved in resolving the issue to better understand what part of the process they are responsible for.
Sub-tasks cannot have sub-tasks of their own. However, if you need to break up a sub-task into smaller sub-tasks, you could achieve this by first converting the sub-task to a standard issue.
Go to “Task” page.
Click the defect id for which you want to create a “Sub task”.
Click “Sub task” button.
Fill “Relate as subtask of issue” pop up.
Click “Save” button.