IDL4CPP — what argument of exception constructor?

In the example code it lists "AnException(int32_t error_code, const char* what)

", but the what argument is not described anywhere, it is not in IDL, where is it coming from?

*Add requirement for implementation of what() method in exceptions *

Specify that the mapped Exception class shall implement the virtual what() method declared by std::exception, which the mapped Exception inherits from, and at a minimum provide a mechanism to provide such explanatory information in the explicit constructor that provides a mechanism to set the value of all exception members.