Subtasks let you split up a big task into a number of smaller tasks which can be assigned and followed separately. Usually it gives a better picture of the progress on the task, and allows each person involved in resolving the task to better understand what part of the process they are responsible for.
Subtasks can be created for any task in any status, except for tasks in the "Completed" status. Subtasks can have references to other tasks, subtasks, and documents. The number of subtasks is unlimited.
Subtasks are created from the parent task screen. Click on the Subtasks tab, specify task name and assignee, and click Create.