The ReportFunction fractionOfSum represents the fraction of the sum of the TargetVariable for two different grouping variables, where the GroupingVariables is used in the numerator and the GroupingVariables except the FractionOverVariable is used in the denominator.
E.g., if GroupingVariables is c("Survey", "SpeciesCategory", "Stratum"), FractionOverVariable is "Stratum", TargetVariable is "CatchFractionWeight", the total catch fraction weight for each stratum is divided by the total catch fraction weight of the survey (for each SpeciesCategory).
Warning! This function is used only to construct an expression to be evaluated in a data.table. Do not use this in R as a regular function like the other ReportFunctions.