Signer Complete callback

I haven't started working on the code to handles callbacks yet. I was looking at http://docs.e-signlive.com/doku.php?id=esl:e-signlive_guide_event-notification and saw the SIGNER_COMPLETE notification. If I created a signer with a custom id how would I get that custom id after receiving this callback? What is sessionUser? SIGNER_COMPLETE This is the notification provided when a signer has completed signing all documents. { "name": 'SIGNER_COMPLETE', "sessionUser": '...', "packageId": '......' }