Hi.
The field "canonical" just must contain the URL that Google should consider "the canonical".
So, for this example:
For example. Google tells that I have duplicate content in following urls: (SEF on, Apache Rewrite ON)
1. www.mysite/my-category/my-prefered-article
2. www.mysite/my-other-category/my-prefered-article
3. www.mysite/my-section/my-prefered-article
Suppose that the first one is the "canonical", you could create an item like this:
URI: /my-prefered-article$
And set for this the canonical:
www.mysite/my-category/my-prefered-article
Two additional words:
1) It was possible to match with a single item the three URLs because they all have a "common part".
2) Be sure that you don't need to match one of these for other meta info. At the moment Tag Meta apply only the first item. But while this behaviour will be preserved in future for ReDJ, Tag Meta will be modified to support "multiple items" in the ordering set.
For more info, ask please.
Luigi