Say I have 2 visitor id attributes, one captures the 'email address' and the other captures the 'Twitter handle'.
I visit a site on my "desktop" and sign up for a newsletter with 'email@example.com'.
Then within the same session I see a product I like and tweet about it using handle 'tealium'.
Then I later go to my "tablet" and submit an order using email 'firstname.lastname@example.org'.
There is no match that will occur, yet.
Then I decide to tweet about my order using handle 'tealium'.
Now a match will occur on twitter handle 'tealium'.
Even though the twitter handles are the same, the email addresses are not. What happens now?
The device visitor profile will all still get stitched together based on 'tealium'.
In the future either 'email address' can be used to stitch to the visitor profile. This only applies because there were two separate devices, each with their own email address, which were later stitched together using the twitter handle.
When the master stitched profile is created, there will be secondary_ids created for Twitter handle and email address. The secondary_id for Twitter handle will be tealium since it's the same between devices. So what will be the value of the email address? Based on the fact that we chronologically order all past events and reprocess them, the first 'email address' captured will be the value of the secondary_id for the master stitched profile - in this case 'email@example.com'.