<error-page>
    <device-type>[ajax|mil]</device-type>    
    <exception-type>ClassName</exception-type>    
    <location>the error page's URI</location>    
</error-page>

It specifies an error page used when an un-caught exception is thrown in updating a ZUML page (e.g., in an event listener). Each page is associated with an exception type, aka, a class deriving from java.lang.Throwable. You can specify multiple error pages, each with a different exception type. When an error occurs, ZK searches the proper error page by examing the exception type one-by-one. If none is found, it shows, by default, an alert message at the client.

The device-type element is optional. If omitted, ajax is assumed. If you want to specify an error page for mobile devices, it has to be mil.