The OnCustomFunction event is fired when the engine has encountered a function name that it does not recognize as a standard function in a filter expression or SQL statement. Assigning an event handler to this event allows for the implementation of custom functions that can be used in filter expressions or SQL statements.
If an event handler is not assigned to this event or the event handler doesn't assign a value to the Result variant parameter, DBISAM will treat the function as if it returned a NULL.