Listbox footers have a variable calculation option that sets the type of calculation to be done with the values in their respective columns. This is useful for doing automatic calculations, such as sum or average, for the row values in a column. These automatic calculations are not supported if (1) the footer's column is based on formulas or (2) the listbox is based on collection or entity selection. For these scenarios, a custom calculation must be used by assigning the value of the footer variable via code. Note that this applies to the command LISTBOX SET FOOTER CALCULATION as well, since it uses automatic calculation.