We have 3 IDs for users in our local Sakai deployment.
Internal User ID - The normal Sakai generated ID, currently passed in "user_id" in launch
External User ID - The Sakai External ID which is currently passed in "lis_person_sourcedid" in launch
Display ID - This is not currently passed in the launch.
We have a local tool which is wanting to know the display ID for a user in an LTI launch.