A Skid Unit is an ATV used in emergencies and rescue missions. They were used by the Shackleton Co-operative in the Phoenix Files series as transport, and they also used them for emergencies.
"A Rescue Mission"[]
Luke Hunter, along with Matthew Reeve and other former Phoenix Security staff, used a skid unit to break into the Shackleton Building (in the book Doomsday) to rescue their families and take down the shield grid with a missle launcher. Officer Kirke drove the skid through the barb wire fence and through the front doors of the Shackleton Building, although it cost him his life.