Sometimes, your content could be directly accesed from a special kind of applications known with different names (news agregators, feed readers, RSS readers, etc.), but with a common feature: aggregate feeds from different sources. More information about that kind of software could be found here: Wikipedia.

b2evolution is able to identify hits that come from links displayed in those applications, and they are summarized in screen and displayed using a bar graph. The daily total should match with the value displayed in the column RSS/Atom of Global Hits Summary.