The EFF wrote a detailed post about UID2, a proposed way to make it easier for advertisers to identify people via their email.
UID2s will be connected to people, not devices. That means an advertiser who collects UID2 from a website can link it to the UID2s it collects through apps, connected TVs, and connected vehicles belonging to the same person. That’s where the “unified” part of UID2 comes in: it’s supposed to make cross-device tracking as easy as cross-site tracking used to be.
But this will also create new incentives for sites, apps, and connected devices to ask users for their email addresses.
Check It Out: Behind ‘UID2’, a Way for Advertisers to Track Your Email