Right as the parent mentions, it only allows grouping among sites that are logically owned by the same entity anyway, if someone wanted to make this work with f

Right as the parent mentions, it only allows grouping among sites that are logically owned by the same entity anyway, if someone wanted to make this work with first-party cookies, they could redirect from myapplication.com to myapplication.owning-party.com (which is what google does with gmail.com -> mail.google.com today) This just allows you to do that while maintaining security boundaries around things like user generated content (e.g. facebook.com vs. fbcdn.net). This doesn't allow some random site that uses third party ads to allow doubleclick cookie access, or vice versa.