This page provides a summary of the feature.
FWIW, in my GA account I have one domain hosted and one user (myself, the administrator). In an attempt to have a little more security, I created a complex, alphanumeric username which I use
only for logins. Then I created a nickname (or two) which I then use for actual email activity. Hopefully, that way anyone trying to break into my account will not only have to know the password (which in my case is over 22 characters) but also the
login username which is
other than the email address that the would-be hacker may be aware of.
Of course, you can also simply have the admin account for administrative purposes only and then create many user accounts for yourself (some for 'alias' purposes), which will forward to your 'main' account.