A sub-task is a 'child' task of a parent task. It is designed by dropping a task directly on top of another task, which indents it on your work console and designates it to be delivered prior to the 'parent' with a prefix 'sub-task'. You will not be able to complete the parent task until all sub-tasks are completed.
Here is an example:
Task A is the parent and tasks 1-3 are the 'child' sub-tasks. When automated, Tasks 1-3 must be completed prior to Task A, however, Task A will appear in it's assigned user's work console at the same time that the sub-tasks are delivered.
Use case: A manager might want to deliver three tasks in parallel to three team members, with the parent task delivered to him/herself as a reminder that each of these team members has a task to complete prior to the manager participating. By receiving that parent task, the manager is put on alert that he/she will have something to do and can monitor the audit trail for the completion of the team member's tasks.
Because tasks are released in sequential order anyways, it is rare that sub-tasks are necessary. Let us know how you plan to use them and we will add examples here!