The 'Local users' sign-in system can be enabled for a website in its Control Panel -> Users -> Module settings -> Allowed methods of users authorization.
When a user signs up to a website with local sign-in system, he/she becomes a user of only one website (the one he/she has registered at). Webtop and User Bar are not available on the website with local authorization. Local users can't log in to other uCoz websites with the same login data, they have to sign up to other uCoz websites again.
Local users log in to the websites by means of a Username (not e-mail) and a Password. uID authorization is not possible on such websites.
Local authorization feature is not a system one, that is why it is a paid service. To enable local authorization feature you need to pay once for one of the paid plans and the feature will be enabled for the whole period of the website existence!