At the top level of the world outline application are roots. Each root contains an outline and some attributes.
An outline is something that can be stored in an OPML file. It contains a nested structure of text and attributes. A collection of text and attributes is called a node, or a headline, depending on the context.
Examples of attributes are type, created, domain, pgfnum, url.
type is the most basic attribute. Most nodes don't have a type attribute. The ones that do give the world outline a clue about how to render the text underneath it. So a blogpost type node will contain paragraphs of text. The text you're reading right now is subordinate to a howto type node.
I would start with a single root. When it gets big you can create another and move some of your work into it. But the outline structure is a very powerful way to organize your web writing and design work. As soon as you start another root you're stepping outside the outline. There are reasons to do it, when your single outline gets big, but until it does -- you're better off staying within a single outline.
You can attach a domain (or cname) to any node. As long as that domain points to the server that your worldoutline software is running on, the worldoutline software will associate that domain with your sub-outline. You can have as many domains in a single outline as you like. This is a real mind-twister, because domains are now pointing into documents. The people who read your site will have no idea how easily you can create these or edit them.
That means that each node with a domain attached to it, the one you get to through the hierarchy, and through the domain that's attached to it.
You can look at the OPML text of any of my outlines. Just look for the white-on-orange XML icon near the bottom of the page. Click the link to view the OPML text. You can open the outline in the OPML Editor by choosing the Open URL command in the File menu. You can see what the attributes are on any node by putting the cursor on it and looking in the About window. (Choose About OPML from the Window menu.)
The OPML source for this outline is at this address.