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
Place strategic search phrases on pages. Integrate selected keywords into your website source code and existing content on designated pages. Make sure to apply a suggested guideline of one to three keywords/phrases per content page and add more pages to complete the list. Ensure that related words are used as a natural inclusion of your keywords. It helps the search engines quickly determine what the page is about. A natural approach to this works best. In the past, 100 to 300 words on a page was recommended. Many tests show that pages with 800 to 2,000 words can outperform shorter ones. In the end, the users, the marketplace, content and links will determine the popularity and ranking numbers.
Many search engine marketers think who you link out to (and who links to you) helps determine a topical community of sites in any field or a hub of authority. Quite simply, you want to be in that hub, at the centre if possible (however unlikely), but at least in it. I like to think of this one as a good thing to remember in the future as search engines get even better at determining topical relevancy of pages, but I have never actually seen any granular ranking benefit (for the page in question) from linking out.
Another excellent guide is Google’s “Search Engine Optimization Starter Guide.” This is a free PDF download that covers basic tips that Google provides to its own employees on how to get listed. You’ll find it here. Also well worth checking out is Moz’s “Beginner’s Guide To SEO,” which you’ll find here, and the SEO Success Pyramid from Small Business Search Marketing.