What do !area and !!area mean in a NET AREA RATIO expression?

Version 1

    !AREA(layername) returns a 1 if there is no data on 'layername'.

    If there IS data on layername, a 0 is returned.

     

    !!AREA(layername) returns a 0 if there is no data on 'layername'.

    If there IS data on layername, a 1 is returned.

     

    They are frequently used to "zero out" or conversely "multiply by 1"

    (leave intact) the result of a larger more complicated expression in a NET AREA RATIO operation based on the existence (or absence) of data on a layer of interest.