The rel attribute of a HTML
<link> element establishes a relationship between two URLs. It can be used to specifying the authoritative version of a page, indicating an alternate version for a different device, language or region, consolidating equity for a paginated series, etc.
Canonical tags are used to help search engines understand which URL has authority over others. This increases the likelihood of important pages being crawled, indexed, and ranked.
In instances of paginated series, rel="prev" and rel="next" tags should be used in order to consolidate equity of individual pages into one URL and resolve eventual near-duplication.
rel="alternate" media="x" tags are
<link> elements used to inform search engines about an alternate URL with the same (or almost the same) content, but specifically designed for other media or devices (smartphone, TV, speech, print, etc.).
hreflang tags help indicate Google and Yandex in what language the content of a page is written and what audience (region) this content is targeting. This allows those search engines to serve the right content to the right user.