The bestseller API returns the products that are likely to generate the most revenue. The algorithm uses a combination of RPV, ATCs and Views, combined in order to maximize Revenue.
The bestseller API doesn't return products according to their conversions only. If you need this behavior, then use the search API and sort the results by best_seller in descending order: &sort=best_seller desc - this will return products sorted based on the sitewide conversions.
If you are integrating JFY widgets on your site, then the bestseller API provides the default content for these widgets until Bloomreach has enough information about individual site visitors to provide personalized recommendations and suggestions. For information about deploying JFY widgets, see Just for you (JFY) API.