Puppet users - Mar 2014 - Puppet3: top scope rooted variable, don't get it

Hello there,
i recently bought the new Pro Puppet 2nd edition.
There is something i don't understand, it's about the syntax of top
scope
rooted variable, here is an example (page 38 of the book):

class ssh_example::params{
  case $::osfamily {
    'Debian': { $sshd_package = 'ssh'}
    'RedHat': { $sshd_package = 'openssh-server'}
    default: {fail("Login class does not work on osfamily:
${::osfamily}")}
  }
}

class ssh_example inherits ssh_example::params {
  *package { $::ssh_example::params::sshd_package:*
    ensure => installed,
  }
}

As the book said: "The variable $::ssh_example::params::sshd_package is a 
way of writing the $sshd_package variable so that it can refer only to a 
single declaration"

I don't get it, OK there is a new package defined at top scope, but where 
is the relation between this new top scope package and the variable from 
the inherits class?
I don't understand the logic, could you help me?

Thanks.

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