PTC fuse resistance characteristic?
One should design a device to work adequately (and safely) with any resistance between RMin and RMax.  In most cases, one would want the resistance to be as low as possible, and so RMax is the most relevant specification.  There are, however, some cases where knowing that the resistance will not fall below RMin may be very important.  Among other things, the RMin will limit peak fault current to (E/R) in the event of a short-circuit, even the supply could output much more.  If RMin weren't specified, it might be necessary to add series resistance to ensure that peak currents stay within allowable limits [even with RMin specified, resistors might still sometimes be necessary, but specifying RMin will allow them to be eliminated in many cases where they're not].