Difference between Tomcat's extraResourcePaths and aliases to access an external directory

Question

Simple question: In Tomcat7, what\'s the difference between using extraResourcePaths and aliases to access an directory outside the application?

I can use either of

Answer 1

After investigating further, I found this difference.

The results of this Java code is different. I still don't know why.

String path = getServletContext().getRealPath("/images");

Using extraResourcePaths, path is below, which is a folder under where Eclipse explodes my web app and not a valid directory.

C:\Projects\.metadata\.plugins\org.eclipse.wst.server.core\tmp2\wtpwebapps\Eclipse_Project\images

Using aliases, path is below and is what I actually need.

D:\path\to\images

Now if someone could actually explain this. :-)