Google announced on Google+ an "important change" to the AdWords reporting, specifically, Google is going to do away with the "Broad match (session-based)" match type label.
Google said around August 24, 2015, they will be replacing the "Broad match (session-based)" match type label with the "Broad match" match type label in the AdWords Search terms report.
Google said it will be removed from all of the reports and all of the historical reports, so if you want to have that data, you must manually download the data before the 24th of August.
Google clarified at the end that "this is a reporting change and will have no impact on Broad match serving behavior."
Forum discussion at Google+.
