Two visitor ID attributes are defined. One visitor ID captures the email address and the other captures the Twitter handle.
A user visits a site from a computer and signs up for a newsletter using the "email@example.com" email address. Within the same session, the user sees a product and tweets about it using the "marketerxyz" twitter handle. The user uses a tablet later on and submits an order with the email address "firstname.lastname@example.org". The user then tweets about the order using the "marketerxyz" twitter handle.
A visitor stitching match occurs on the twitter handle. The twitter handles are the same, but the email addresses are not. What happens now?
The two profiles from the two devices are stitched together based on the Twitter handle.
When the master stitched profile is created, two secondary IDs are created, one for the Twitter handle and one for the email address. The Twitter handle secondary ID is set to "marketerxyz" because the handle is the same on both devices.
During visitor stitching, all past events for the profiles are chronologically ordered and reprocessed. The email secondary ID is set to the first email address encountered during reprocessing, in this case "email@example.com".
In the future, either email address can be used to stitch to the visitor profile. This only applies because there were two separate devices, with different email addresses and profiles, which were later stitched together based on the Twitter handle.