They have a database of everyone's rental car pickup and destinations. Let's say a car that has NJ plates is currently located in Florida and needs to get to NJ in 3 months. They would try to give that car to people doing a one way rental to go north, eventually finding its way to New Jersey or somewhere close to it like PHL. For example, it would match up 3 renters: one going from MCO-ATL, one going from ATL-DCA, and one going from DCA-EWR, and give them all the same car