Any headline in any outline can link to another outline located anywhere on the Internet. When you expand the headline, the OPML Editor fetches the outline that's linked to, and adds it underneath the original headline, as if the linked-to outline were part of the outline doing the linking. In this way you can include my work in yours, much the same way your web site can link to mine. We call this kind of linking inclusion. And of course when the included outline changes, there's no need to modify the outline that's doing the including, it adjusts automatically the next time you collapse and re-expand it.
Note: Some people call it transclusion, which sounds fancier, perhaps, but it's confusing to have two names for the same thing, so please let's call it inclusion.
To create a Link node in any outline, right-click on the headline that you want to link from and choose Add Link from the popup menu. Enter the URL you want to link the headline to and click on OK.
If you want to see a Link in action, open the states.opml outline, above, and navigate to Florida. The up-pointing arrow indicates that it's a link. Expand it by double-clicking on the link, to see some places in Florida. They come from my Florida outline. Screen shot.
Linking is the key feature that makes delegation work in the podcastingj community's directory. When I link to another OPML directory, I'm delegating authorship of that sub-directory to someone else, who could be anywhere in the known universe that OPML is supported.
An important limit -- to include an outline, its name must end with ".opml." If it doesn't the OPML Editor will open the linked-to item in the web browser.
You can nest inclusions as deeply as you want. In other words it's OK for an included outline to include other outlines.
In addition to OPML files, you can link to RSS 2.0 feeds, in exactly the same way, and they will expand in place.
There's no top level to the World Outline, any more than there's a single home page for the World Wide Web. It's a chaos of competition, and that's why it works!