I don't know if this is old news but it's new for me. I did a search for alibaba on google. The site alibaba.com turns up with included site links but it seems that google has now added a search feature directly under the link. You can search alibaba site from the google search engine. See screenshot here Google's new search feature
I have seen that on other sites as well recently it appears that this feature is on large traffic volume sites only as far as I have noticed such listing may also depend on the number of search queries made ON THAT site using G search
Its old feature, usually Google shows such results for top sites which are having heavy traffic, thus gives them more importance
Agreed with you mdamin76 yeah google include this feature to become more user friendly , n it will help a user to find out that how much result does a particular keyword have