Windowing transforms intensities from one range to another, for example from 1000-1900 into 0-255 in order to display CT pixels on regular screens. Values outside the range map to endpoints: -1000, 0, 200 and 999 all map to 0, while 1901, 3000 and 25000 all map to 255. Window is the width of the range (901 in the example above) and level is center of the range (1450.5 in the example).
Therefore, window and level need a range to exist, they are not defined for a single value.