username

    Optional. Required for detailed information about a user.

    Specify the user login name. For information about a user
    on your local system, do not include the @hostname. For
    information about a user on a remote system, include the host
    name (username@hostname).

 @hostname

    Optional. Required for information about users on a remote
    system.

    For information about a specific user on a remote host, include
    the user name with the host name (username@hostname). For
    information about all users on the remote host, specify the host
    name only (@hostname). Omit the host name to display information
    about users on your local host.