Overview
A 404 error can happen within two contexts, at the server level and application level. A server level 404 error is not a common occurance, the most common would be when there is no application or object mapped to "/" path. If an application or an object is mapped to "/", a 404 error would occur within the application context and not the server context.
Axiom provides a default 404 handler, however it's very primitive, just giving the user an error stating that there is "No application on this server matched this request." and a list of available Axiom applications. This may not satisfy your needs, therefore Axiom provides an interface to override the default 404 handler. Create a file named default.html and place it your $AXIOM_HOME/apps directory/. Now when a server level 404 error happens, the server will then display that file as opposed to the primitive default 404 error page.
For more information on how to handle an application level 404 error, please refer to Custom Error Handling