when a user logs in, their ID is stored in a session variable.
on an update page, you would want the Unique Database value to accept the email address for the logged in user but not any other email address that is already in the database.
by setting the Key Value to the user session, if a user is logged in, it will look up the logged in email and accept that.
in your case, this is a registration page, not an update page, so you dont want to allow the logged in users email.