DPNSS trunks are a special kind of inter-system networking trunk, like QSIG in a way... You would only apply that to a route that is a DPNSS networking link where you access the PSTN via that link. In working with this thing since 2001, and on prior SX-2000 systems, I have never set this up or used it. Your route should be blank for a "normal" route, and Emergency for all Emergency/PSAP calls.
FYI... Open the embedded help and search "DPNSS" and you can read more about it, but basically it is depreciated and when it was used it was very special purpose applications.
