Search Engines has co-occurring demands database
Latest search engines evaluate quantity and quality of information. Reputation of the keywords is nothing to do with ranking.
In order to check the quality of contents, they build database which has relevance information about user's interest, in addition to the standard dictionary of keywords.
The standard dictionary has semantic data, but it do not contains information about experience of users. In addition to them, search engines build own database of keyword's relationship.
Theme is just a collection of keywords. It lead to semantic information bigger than keywords in it or other abstract keyword.
There are small theme and big theme. Small theme is concrete and big ones are abstract. The theme represents semantic information about abstraction level.
On actual experience in our life, several themes are included in a demand. The search engines log the query and build database of co-occurring theme and keywords.
Actually, the database consists of keywords. There are keywords which is included in multiple theme, they are interface keywords of the theme relationship.
The bing search engine shows co-occurring keyword on the SERP.
When you create content, I think you do not have to stick on the co-occurring keywords, but you should consider to use some keywords, and check which keywords the search engine can detect.
By using the Keywords tool on Google Adwords, you can check it.
By using the keyword tools, and input the landing page into the text input form, then you can check which theme the search engine can detect at the page.
Then you have to check multiple small themes about the page are detected. I guess you are writing a page for single big theme, and the theme contains some small themes and co-occurring themes.
You have to check these co-occurring themes are detected by the keyword tool.
And you can view the keywords in the theme.
By looking at this list, you can find co-occurring keywords easily. On this example, the "search engine optimization" is the base words, and "company", "services", "specialist", "uk", "sydney" are co-occurring keywords.
If you can include such words naturally in the content, or add link to the page which mentions those words, you had better do that.
That is because by doing that, you can aim at long tail keywords which is suitable for your contents. The search engines has great system and they can detect similar keywords. But by including them as text, they can detect them easier.
But you have to choose co-occurring keyword naturally, and don't do co-occurring spam. There are a lot of co-occurring keywords, therefore you can write and include them naturally.
The density and iteration of the keywords is nothing to do with the ranking, now a day. The search engines become much wiser than before.
They evaluate websites with quality and quantity of information. Especially quantity of information is important.
If there are many pages in the website has links to the page, which has co-occurring theme, they think the website has correct and arranged contents. And the quantity is much, they appreciate the website high.
Therefore, including effective and natural co-occurring keywords is essential technique for SEO writers.