In California non op is a vehicle that has been taken off the road for various reasons by the owner. Telling the dmv that your vehicle will not be registered for the length of time this vehicle is off the road. When the time comes to register it again you only pay the current fees. Not all the years of back fees it was not registered.
Example, I recently heard an old friend of mine is out of the area for awhile. Myself and his family are going to clean his property. There sits his 1999 Ford contour svt, 1985 Ford F-150 5.0 both vehicles have sat since 2012. We plan to get these two Fords going again. When they are completed and road worthy the registration will only be the current dmv fees. Both were put into non op back in 2012. No back fees to pay.
Whoever buys this stx Ranger will owe no back fees on tags.