System Task allow you to execute a task list against a document that is not necessarily associated with a life cycle. You must associate the system task with any document types on which you want to execute the task.
System Tasks configured with the Set Property Value action and the Work Item Property option selected will not execute properly.
System tasks that require user interaction are supported in the Unity and Web interfaces. System tasks that require user interaction are not supported in all other Core-based interfaces. When using the Classic Client interface, it is not recommended to configure system tasks that require user interaction.
These types of task lists differ by when the task is executed and which computer executes the task on the document.
The same task cannot be placed in both Ad Hoc User Tasks and Web/API Tasks folders. Web/API tasks, however, can be a part of any other task list (including Ad Hoc task lists, User Work, etc.)