QUOTE: “Supplementary Content contributes to a good user experience on the page, but does not directly help the page achieve its purpose. SC is created by Webmasters and is an important part of the user experience. One common type of SC is navigation links which allow users to visit other parts of the website. Note that in some cases, content behind tabs may be considered part of the SC of the page.” Google Search Quality Evaluator Guidelines 2017
Being ‘relevant’ comes down to keywords & key phrases – in domain names, URLs, Title Elements, the number of times they are repeated in text on the page, text in image alt tags, rich markup and importantly in keyword links to the page in question. If you are relying on manipulating hidden elements on a page to do well in Google, you’ll probably trigger spam filters. If it is ‘hidden’ in on-page elements – beware relying on it too much to improve your rankings.
QUOTE: “For the mostpart it should be fine I think the the tricky part that you need to be careful about is more around doorway pages in the sense that if all of these pages end up with the same business then that can look a lot like a doorway page but like just focusing on the content duplication part that’s something that for the most part is fine what will happen there is will index all of these pages separately because from  from a kind of holistic point of view these pages are unique they have unique content on them they might have like chunks of text on them which are duplicated but on their own these pages are unique so we’ll index them separately and in the search results when someone is searching for something generic and we don’t know which of these pages are the best ones we’ll pick one of these pages and show that to the user and filter out the other variations of that that page so for example if someone in Ireland is just looking for dental bridges and you have a bunch of different pages for different kind of clinics that offer the service and probably will pick one of those pages and show those in the search results and filter out the other ones.
There used to be a time where you could add a lot of keywords to your pages and posts, do some old-fashioned keyword stuffing, and you’d rank in search engines. But a text with a lot of the same keywords in it is not a pleasant read. And because users find this kind of copy terrible to read, Google finds it terrible too. That’s why ranking in Google by doing keyword stuffing, fortunately, became hard to do.
So you have a new site. You fill your home page meta tags with the 20 keywords you want to rank for – hey, that’s what optimisation is all about, isn’t it? You’ve just told Google by the third line of text what to filter you for. The meta name=”Keywords” was actually originally for words that weren’t actually on the page that would help classify the document.
In Chapter 2, we learned about SERP features. That background is going to help us understand how searchers want to consume information for a particular keyword. The format in which Google chooses to display search results depends on intent, and every query has a unique one. Google describes these intents in their Quality Rater Guidelines as either “know” (find information), “do” (accomplish a goal), “website” (find a specific website), or “visit-in-person” (visit a local business).
The higher the search volume for a given keyword or keyword phrase, the more work is typically required to achieve higher rankings. This is often referred to as keyword difficulty and occasionally incorporates SERP features; for example, if many SERP features (like featured snippets, knowledge graph, carousels, etc) are clogging up a keyword’s result page, difficulty will increase. Big brands often take up the top 10 results for high-volume keywords, so if you’re just starting out on the web and going after the same keywords, the uphill battle for ranking can take years of effort.
In Keyword Planner, formerly known as the Keyword Tool, you can get search volume and traffic estimates for keywords you're considering. Unfortunately, when Google transitioned from Keyword Tool to Keyword Planner, they stripped out a lot of the more interesting functionality. But you can make up for it a bit if you take the information you learn from Keyword Planner and use Google Trends to fill in some blanks.
SEO may generate an adequate return on investment. However, search engines are not paid for organic search traffic, their algorithms change, and there are no guarantees of continued referrals. Due to this lack of guarantees and certainty, a business that relies heavily on search engine traffic can suffer major losses if the search engines stop sending visitors.[61] Search engines can change their algorithms, impacting a website's placement, possibly resulting in a serious loss of traffic. According to Google's CEO, Eric Schmidt, in 2010, Google made over 500 algorithm changes – almost 1.5 per day.[62] It is considered a wise business practice for website operators to liberate themselves from dependence on search engine traffic.[63] In addition to accessibility in terms of web crawlers (addressed above), user web accessibility has become increasingly important for SEO.

Keywords are as much about your audience as they are about your content, because you might describe what you offer in a slightly different way than some people ask for it. To create content that ranks well organically and drives visitors to your site, you need to understand the needs of those visitors — the language they use and the type of content they seek. You can do this by talking to your customers, frequenting forums and community groups, and doing your own keyword research with a tool like Keyword Explorer.