A subworkflow is any workflow that is referenced from within another workflow. Use subworkflows to model complex business processes into logical, more manageable, and reusable subprocesses.
You can drag a subworkflow from the Workflow Step Sources window and drop it onto the Layout tab. When the package, request, or release reaches the subworkflow step, it follows the path defined in that subworkflow. The subworkflow either closes within that workflow or returns to the parent workflow.
Subworkflows are defined in the Workbench using the same process as when configuring a workflow. When creating a subworkflow, be sure to set the following:
Set the Sub-workflow option to Yes.
Make sure that the validation for the step leaving the subworkflow layout matches the subworkflow step in the parent workflow.