A task series allows you to accelerate task completion using a hierarchy.
Each level you create establishes the hierarchy for that series. The task series hierarchy starts with Level 1 and must contain at least two levels. The users you assign to a level are mandatory assignees. As you create additional levels, the users on these higher levels become optional task assignees for all prior levels. Any mandatory or optional assignee can complete a task.
When a mandatory user completes a task, the next task in the series appears. If an optional task assignee completes a task, that assignee's level in the hierarchy determines whether the next task in the series appears or if the series is complete. For example, when the top level assignee completes a task as an optional assignee, the task series is complete and no additional tasks appear. In addition, when an optional task assignee completes a task, the mandatory task for that user is also complete. When you create a task series hierarchy, you can assign levels to users only and each level must contain unique users.