An expression function is a function that can be used only inside expressions. It takes one or more arguments, where each argument is itself an expression, and produces a single value as the result.
For example, the expression function Sum (see available expression functions in BasicExportFunctionSet) can take an arbitrary number of numerical values and return their total.
Template:
The sum of values from 1 to 5 is $(Sum(1,2,3,4,5)).
Result:
The sum of values from 1 to 5 is 15.