Primary Extension (looking in database 8.5.1)

Version 1
    This document was generated from CDN thread

    Created by: Ron Buchanan on 02-11-2011 11:10:55 AM
    Is the primary extension associated with the device itself?  Or only thru the user to the device?  I've found tkDnUsage in endusernumplanmap and applicationusernumplanmap, but I've found it nowhere else and nothing to indicate that there is a device-level primary extension.
    Any help will be appreciated.