When considering Range vs. Radius, they are the same.  Think of a circle, with the beacon in the center.  The Bluetooth signal emits from the beacon in all directions (omni-directional).  The signal is stronger at the beacon, and weaker as you move away.  Eventually, not signal is found.  

The "signal" is radio and is affected the same way as all radio waves, absorbtion, reflection, bounce yielding an approximation of distance.

The approximate distance is described as "Range" or "Radius", both are equal.  The value answers the question .. "how far from the beacon will I still received a viable signal".

If the beacon is in the center of a large blurry circle, and the outer edge represents the weakest beacon signal that is viable, the distance from the beacon to the outer edge is the Range or Radius.

Related Links:
The beacon manufactures describe this technically as RSSI.  
Relationsip of power to range.