This is only available in the MSIE browser. It is useful if you are developing scripts and it is possible that you could build a reference to this into a form that is submitted as part of an error handler. That way you might determine what the error is, what caused it and note the build number of the script interpreter. This may then yield a pattern. An essential part of the fault diagnosis process involves the search for a pattern in the failures of a system. It is very possible that a certain build of the interpreter could manifest a bug which is not present in other builds.