What are composite and virtual mail folders?

Hi, I'm trying to understand what "composite" and "virtual" folders are as they relate to email folders in Usermin. I was not able to find any documentation searching the site and was only able to find this post (http://www.virtualmin.com/node/9331) on the topic.

What are composite folders and how are they used?
What are virtual folders and how are they used?




A composite folder is a folder that contains two or more others, but combines all their content so that it looks like a single large folder.

A virtual folder is one that contains links to messages in other folders. If a message is deleted from a virtual folder, the original will not be deleted. The "Search results" folder is an example of this. However, if you create your own virtual folder, you can "copy" additional messages into it.