Sub-services give you the ability to connect services in a parent/child relationship. This capability could be used to create services such as a "follow-me" type of Universal Service or a Dedicated service that has child services associated with it.
Each parent service must specify which service types can be allowed as child services. This is accomplished when you set up the service in Product Catalog.
When you assign a parent service to a customer, the Order Wizard presents you with a page to select the allowed child service types.