The if function returns one value if a test evaluates to TRUE and a different value if the test evaluates to FALSE.
if(test, true_value, false_value)
test is a value or expression that evaluates to TRUE or FALSE.
true_value is the value that is returned when test is TRUE.
false_value is the value that is returned when test is FALSE.
if(not IsNull(SoldQuantity), SoldQuantity, OutputQuantity)