Have you ever searched for something and saw most of the top results are from the same website? This might be unimportant to some, but many want to see a wider diversity of results. Google recently announced that it has updated the SERP to show a more diverse set of search results. How so? Google has stated that it will aim to show no more than 2 results from the same domain (What domain name is) in a particular query. The company also added that it will show more than two results from the same site in some cases when the results are particularly relevant to the query.
In general, Google will treat subdomains as part of the main domain, thus treating the listings from subdomains as it’s been originated from the same site and will be counted towards the two results. Nevertheless, Google reserves the right to treat some subdomains differently by stating “However, subdomains are treated as separate sites for diversity purposes when deemed relevant to do so”.
It is also important to note that this update will impact the core results ONLY, thus not affecting the additional search features such as top stories, featured snippets, image carrousels, map listings, etc.