The general idea here is that you want to redirect visitors if a particular file is missing; however it is considered better to simply state the requested URI is missing and issue a 404 Not Found. Some site developers may choose not to do this and this method will provide an alternative and a variety of choices.
Issue a non-404
In this example, example.com should, of course, be replaced with your own website address. We want to issue a clean web page to outsiders and those with an empty referer but sill allow internal people to get the standard 404 Not Found error message.
Replace the Missing File with Another File
If you want the missing file to be replaced with a URL from another site, you can do so by providing a full URL (http//:some.other.server/) in that RewriteRule:
This is the same as
Issuing a non-404 (file found but not the one requested) is a way to cleanly fail if site content if missing. However this can confuse and polute the web crawlers indexing your site.
Issuing a Redirect-If-Missing is another approach however this too can lead to later confusion about which resource is valid if not up-to-date.
Bottom-line: Choose a particular error eporting and handling policy and stick with it.