six classes of services in delay tolerant networks

The delay tolerant network envisages six classes of services:

1. Custody Transfer.
2. Return Receipt
3. Custody Transfer Notification
4. Bundle-Forwarding Notification
5. Priority of Delivery ( Bulk, Normal, Expedited )
6. Authentication( sender's identity & integrity of  bundle content )

what made confused is that why custody transfer and custody transfer notification are two classes of services rather than one class.