I'm trying to figure out how to use it. I understand that this tag tell Google which page is the root parent / parent of the current page.So, for example, on a page that's listing a product from a certain category, the rel=index value should be the product category page (the page which lists all products from that category). Am I right?And the index value of that product category page is the Products page, ie. the page which lists products from all categories (or featured / most bought / popular products or whatever).Is this how index should be used?If yes, then what if the page which the index points to has noindex, follow, because it serves duplicate content?
About the  tag
seo;html;noindex;rel
You're correct in your understanding. The rel=index is designed to point bots to the index for the current document, which could be a category listing, a monthly archive, or the root of the site itself.If the page designated as the index is marked noindex, follow, it's because the website maintainer wants search engines to ignore the listing page, but to still follow all of the links it finds and index those.Why would they want this? Take your example of a product page, pointing to its index which is a product category listing page:/product-category/example-product/       ^                ^ category index        pageThe product page is full of keyword-rich content and can have hand-crafted titles and descriptions. The category index, on the other hand, is dynamic and hard to optimise for SEO. And even if a visitor lands on the page from a search engine, they won't necessarily have the correct context for the site, so they won't have the best user experience. So it makes sense not to index [noindex] the page.But the category index links to all of the other product pages in that category, so it's really valuable to have the search engine follow all the links [follow]. All the product pages that the bot finds will be marked index and optimised for SEO, so they should perform well in the search engines.