Extensibility point to add new context header information. Please refer to Sonos documentation for details It is a token that is specific to a device to allow that device to stream. If a deviceSessionToken was returned in a previous call, this is provided here. There may be some tracks that do not include one in the mediaUriResponse; this will not cause Sonos to remove the previous deviceSessionToken. Sonos will send this element until a new one is returned, always. This element can be used to avoid having to re-validate the cert by encoding that information. It can provide a lower cost short cut for encoding session keys.