The sessionID parameter sent on each event is used to determine session lengths and session counts. The number of sessions for a player is a count of the number of distinct sessionID 

that the player has recorded. 

The deltaDNA SDKs will generate a fresh sessionID each time the SDK is Started. The latest SDKs also contain a NewSession method that can be used to issue a new sessionID so you can control the session timeout / regenenertation more granularly as the player moves between background and foreground states.