What does canonical URL say
Canonical URL says that although there may be multiple URLs with this content, you should treat the canonical one as the URL of this page.
Which should I use for
If you would only want one of the pages to come up when you are searching, because they are identical, or near identical, then you should use the same canonical URL, otherwise use different ones.
Uses for canonical URL
In a lot of these cases redirects could be used instead, and are usually preferable.
- Setting if http or https shows in searches
- Setting if www should be used or not
- Pages that can be accessed multiple ways