I'm going to update the plugin description (including all new features) this weekends. Canonicalization helps webmasters to avoid multiple versions of the same content on different URLs. It's very bad for SEO and can lead to bad consequences like:
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- Search engines don't know which version(s) to include/exclude from their indices
- Search engines don't know whether to direct the link metrics (trust, authority, anchor text, link juice, etc.) to one page, or keep it separated between multiple versions
- Search engines don't know which version(s) to rank for query results
- When this happens, site owners suffer rankings and traffic losses and engines suffer lowered relevancy.
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Both options solve this problem. Redirect option makes 301 redirect if duplicate content is detected and rel option adds special tag which provides content original URL.
If you have any other questions feel free to ask.