It designates a vehicle with certain factory modifications aimed at "essential services" such as fire, police, SES, military police, rural fire service, Customs etc.
- always for a government (local, state or federal) and not sold to the general public. They may have one or many modifications such as lowered suspension, sump guards, extra wiring for radio etc. My BT1 must have been looked after as it was immaculate when I bought it (at top dollar) but many have been thrashed as pursuit cars etc.