You see two if you have board email deactivated and 4 if you don't.
disabled - no one can register
none - people can register and have instant access to the board with no account verification
by user - the user will get an email sent to the address used at registration and they'll have to click a link in that email to verify their account
by admin - anyone with the admin permission of 'can manage users' will get an email where they can check an account before activating it with a link in that email.