The complement systems are like an odometer on a bicycle. 

The odometer in car won't move back. But in bicycle,if it goes backward, the odometer will also moves backward. 

Consider an odometer with three digits, if riding the bicycle start at zero and end with 650, it is not possible to identify that the bicycle moved forward for 650 miles or moved backward for 350 miles.

To remove the confusion, the easiest way is to split the numbers into two parts. Use the first part from 001-500 for positive miles and the second part from 501-999 for negative miles.

Now if the odometer reads 983, it means the bicycle moved 17 miles backward but not 983 miles forward.

The numbers 501–999 in the second part represent the radix complements of the numbers 001–500 in the first part.