My developers were curious about how RoboHelp HTML outputs run searches.
They noticed that a search for a plural word, such as "lists," returned both singular and plural versions of the word. In other words, it returned both "lists" and "list."
These results are consistent for all words ending in "s."
The developers are concerned that users are getting too broad of search results returned.
Is this by design?
If you are referring to "HTML output" as being a Microsoft HTML Help file in CHM format, there isn't anything you can really do to influence the behavior.
If the output is WebHelp format, look at your Single Source Layout recipe. On the left panel is an item labeled Search. Click that and you will find options that influence the search.