Useful RSSI range?

It is the difference between the max RSSI reading and min RSSI reading. Each RSSI reading will have a fixed offset which is based on the reference design. Depending on the offset on your design you will get different absolute reading. However the difference between max and min RSSI will always be same.

In the datasheet it take the CC2540EM into consideration and depending on the gain mode the RSSI range is -99 to -44 or -99 to -35

But what of the keyword "useful" what makes the range of RSSI we experience "useful"?

Well the usefulness of this can be in applications like proximity detection, based on the relative strength of the device with regards to a signal source.  

Is "useful" equivalent to "differential" in this case? :)