A sub-task issue is typically one of many tasks that make up a story (or parent issue).

To create a sub-task:

1. 
2. If you are using Scrum and the issue is currently in the planning phase, click Plan; otherwise click Work.
3. Locate the parent issue (story) for your new sub-task, and click its issue key (or type t in Work mode) to display the Issue Detail View on the right-hand side of the screen
4. Click the sub-tasks icon:
   
5. Click Create Sub-Task.
6. Enter your sub-task's details into the Create Issue dialog box (see Creating an Issue).