The Tealium Swift library generates a randomized and unique persistent visitor ID the first time it is launched with the visitorId property. This value is included in each tracked event as the attribute tealium_visitor_id. This value is the primary visitor ID used in Tealium AudienceStream. Include secondary visitor ID attributes as needed by adding them to the persistent data storage.
If you are using AudienceStream, this becomes the main first-party visitor ID that’s associated with this visitor. You may provide additional known visitor IDs as needed, by sending them as variables in your tracking calls.