This read-only Counter64, in sessions, indicates the total number of SharePoint Optimized HTTP sessions, incremented for every session on which SharePoint optimization can be performed, where a session is tagged as SharePoint based on information in the HTTP request. An admin uses this to see how much of the HTTP AO's activity is specifically attributable to SharePoint traffic versus general web traffic. A zero or flat value here in an environment known to use SharePoint heavily would suggest SharePoint-specific optimization isn't being triggered as expected.