SEO engines are ran by what are called robots that are guided by a few hundred algorithms to determine the ranks that web sites obtain for specific keywords. There are many methods that can be used to get a high ranking, but it is usually more than just tactics that will get a high ranking.
Placement has a role in how the robots determine the scores that are used in SEO ranking. Having the primary keyword in the URL will have a high bearing on how the site is ranked. This is the root to the full website and is also shown in each and every page that is listed in the site. Usually the URL will be the brand name to the company or a common logo that has the keyword in it.
Metadata is another factor that has a rather high value in the SEO rankings. This is the data that is part of the signature of the items and the pages of the website. Many sites will use these as tags to contain the primary and secondary keywords for the robots to locate and to use in the ranking process.
Coding in the page itself can play a good role in the SEO process. The robots will even take some value to the remarks that are used in the coding. These have been commonplace for intentional mistyping of the keywords so that they can play a role in the SEO. Some sites have made this a habit in the secondary pages of the site as they have a lesser role in finding customers and web surfers, yet gives an additional value to the pages. Some would deem this as a waste, but it has developed minor results to some sites that would have otherwise been overlooked.
The core of many of the pages is in the actual text. This is where a number of advanced methods have been developed to help in the keyword saturation. Many times a website will use a primary keyword or key phrase and up to three secondary keywords or key phrases. The typical saturation rate of keywords is three to four percent for the primary keyword and one to two percent for the secondary keywords. This can be quite simple if the site is only offering one service, but can be challenging if the site is offering multiple services or products.
Some sites have also included a blog or comments page where they are designed to have the keywords in the text. These pages are usually devoted to this sole purpose and can actually have quite positive results in some search engines that place a value in the number of keywords versus the percentage of saturation.
One thing to remember with topic sentences and also headers is that the robots often see them as both body text and also as a form of metadata. This gives them a dual value in the area of keywords and in proper SEO optimisation.