One of unique features of new Helix3 is allowing to use error position which can be used only on 404 error page. In general an Error 404 means that requested a page that does not exist. This could be because user have entered the wrong URL or that the address that user have requested references resources that have been moved, for example; you changed the aliases for menu items in Joomla 3.4 and Google didn’t notice this yet. Another possibility is if a website has moved a page or resource but did so without redirecting the old URL to the new one, especially after migration. When that happens, you’ll receive a 404 error instead of being automatically routed to the new page. Now you can help a little bit more yours "lost" users.
All you have to do is add new module(s) and publish it on "404" position. It can be very useful, because you can add there search module, menu or custom code module.
Everything depends on you.