What does "base" link mean here?

[nicole.healthline]

On http://www.google.com/support/webmasters/bin/answer.py?answer=139394, it says:

rel="canonical" can be used with relative or absolute links, but we recommend using absolute links to minimize potential confusion or difficulties. If your document specifies a base link, any relative links will be relative to that base link.

Where would a document specify a base link? And how?

[RyanKent]

A web page can provide a line of code in the area of the page which defines a base URL. Example:

<base href="http://www.mysite.com/">

The remaining links on the page will prepend non-absolute links with the base URL. If you wish to refer to a URL: "http://www.mysite.com/cars/ford/mustang" then you could simply use "cars/ford/mustang". This method is referred to as relative URLs. The URLs are relative to their base.

Offering the full URL is referred to as an absolute URL.

[Bevelwise]

I would always recommend when implementing canonical tags that you use an absolute links, which just means your entire url starting with http://

When they are talking about relative canonical links they mean relative to the "base" link, which could also be called the root of your website.  So if your url was: http://www.rootdomain.com then your relative rel canonical tag would be