%CALC{formula}% variable is handled by the SpreadSheetPlugin. Over 100 functions are available, such as $ABS(), $EXACT(), $EXISTS(), $GET()/$SET(), $IF(), $LOG(), $LOWER(), $PERCENTILE(), $TIME(), $VALUE().
%CALC{formula}%
%CALC{$SUM($ABOVE())}% returns the sum of all cells above the current cell
%CALC{$EXISTS(Web.SomeTopic)}% returns 1 if the topic exists
%CALC{$UPPER(Collaboration)}% returns COLLABORATION
%INCLUDE{%CALC{...}%}%) because it does not get evaluated inside-out & left-to-right like ordinary TWiki variables. Use CALCULATE if you nest variables.