I believe that it means the range will be increased by "rmb" when the target moves out of the attacking range.
Imagine this: Unit A has 1500 range and 600 rmb
Now unit B enters the 1500 mark and is attacked. Now it tries to flee, and flees up to range 1740 which actually is out of the attack range of the unit A. Still, unit A may attack unit B, becouse its rmb and range make 2100 together, so the actual range would be 2100.
I'm not 100% sure but I believe that's what it does. Hth, Davey