The "primary" will have a /u/0/ in the address. It's the one you logged into first if all accounts were logged out. It's the one it defaults to if you go to oth

The "primary" will have a /u/0/ in the address. It's the one you logged into first if all accounts were logged out. It's the one it defaults to if you go to other google services (plus, youtube, etc). Nothing exclusive about it, except that you logged into it first sequentially.