The point of user defined messages is
1) You can include far more relevant detail about the cause of the problem. For instance I have a number base conversion routine which raises an error if the character isn't valid for the number base. If you try to convert "678" from octal to decimal an error message saying "The character '8' is not valid for the base 8 number system" is much more useful than error 1804 "The specified data type is invalid" which was the closest I could find.
2) You don't have to search through all the messages trying to find one that fits and makes sense.
3) The error messages can be tailored to your local language(s)

Now, all of the above *can* be achieved with Kix as it is, using global error variables and so on. In the Kix community we could decide to adopt a standard - our UDFs should always set / reset the global variables $_UDF_ERROR and $_UDF_SERROR for example. This would give the same features, and if adopted as a standard would allow portable function libraries to be distributed. The point is, why do it? If a future release can support user defined error messages through the @SERROR macro let's do that.

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Richard Howarth