The Stack's core transaction processing architecture uses a SIP message-based interface between the Transaction (and lower) layers and the Transaction User layers. DUM is one example of a Transaction User.
This diagram shows the reSIProcate Transaction and Transport Architecture. Common colours indicate related, thread connected components. File:Http://scm.sipfoundry.org/rep/resiprocate/main/resip/stack/doc/ReSIP diagrams - Architecture.png
NOTE: The diagram was updated at a recent (2005Q2) documentation session and includes the TU selector now. Unless someone needs a pretty version again, I'll defer to the controlled PNG as the architecture document.