Site Diversity change means that not more than 2 pages from the same domain or subdomain will appear in search results for the same search query. However, there can be cases when Google finds a domain highly relevant to the search query and show more than 2 results as well.
This brings a really good opportunity for small and medium businesses to increase their first page rankings with right SEO efforts which were earlier captured by some big brands.
This change comes in the month of June, but it is not a part of Broad Code Update. We are still waiting for Google's June Broad Core Update & will expect it within the same month.