Difference between a VoIP Service DN and a Trunk DN

[Scottyjohn]

We have a site that is configured to send calls going to agents to an egress voice gateway at the premise site, and the SIP Server is configured to use a DN of type VoIP Service to get the call to the contact of the gateway.  We also have a DN configured as type trunk which is used with a prefix of 9 sending outbound calls to a different gateway for routing out to the PSTN.  I need to know why the egress gateway route would be configured as a VoIP Service rather than a Trunk?  Any ideas as to what the differences are that would make this necessary?  Also, in selecting a destination for a call, does SIP Server try to find a VoIP service with a matching prefix before it looks to find a trunk with a matching prefix?

Any advice you can give would be much appreciated!