Hi Jordy,
I might be wrong but I believe these functions are meant to be used from Java actions inside an "Core.retrieveXPathQuery" function call. From the modeler you can use the "Aggregate list" action.
-Andrej
here is an example:
Core.retrieveXPathQuery(ctx, "sum(//Sales.Order/TotalPrice)");
EDIT: the id function seems to be used internally whenever you do a retireve from database by id, see the example here https://docs.mendix.com/refguide/xpath-id