Some search engines will match on partial words found within a larger word. This is often referred to as "word stemming." For example, with partial matching turned on, the phrase "computer program" would find a match within "computer programmer."

When you select a search engine on the Page Critic, you'll be prompted on whether partial matching is believed to be supported by the search engine. If it is supported, then it's best to analyze the page with partial matching turned on to get more realistic results.

For example, if you use marketing in your Title tag, a search on marketing or market will yield a match on engines that support partial matching ("word stemming"). However, words like companies will not always yield a match on company since company is not an exact "substring" of companies. In these cases you'll want to try and use both forms of the word.