If you search for a keyword in the global search (ex: in the searchfield in the top-right corner of this site), you will get results from wiki pages and tracker item descriptions, but also from tracker items that do not mention your search keyword _but_ have some other tracker items in the "Related" field.
I'm under the impression that this should not be the case, because it leads to a ton of false-positives and buries the needle in the haystack.
When running a global search, I'm guessing some not-really-contents fields should be excluded (unless doing an advanced search with criteria about which fields are used to find what string), to prioritize titles, descriptions, and comment contents (without the @username_mention strings).