Custom error setting is a web server setting that sets your website Error Pages, but you can still get a generic error message if you manually specify your custom error setting in your web.config. In this case all error caused by your ASP.net will still use your ASP.net web config configuration.

But the other file types such as PHP, JSP, or even ASP will use your web server Error configuration.For more information search on google for Custom Error Pages in ASP.net keyword.