If you mean 'range' on the trip computer, what happens is that as you ride down the road the average economy increases from say 35mpg to 40mpg. The trip computer multiplies this by the measured fuel in the tank and generates a range to zero.
I find when I fill up [and i reset the trip counter B], that i'll cover 35-50 miles [less if thrashing it] and the range will stay at about 130 miles, then it starts to dropping at that point.
When is says range is zero there is 10-20 miles left [maybe more], but I avoid that as pushing motorcycles is a kids game.