When you turn on external communications, you can define which specific organizations that use Lync that you want to federate ...

When you turn on external communications, you can define which specific organizations that use Lync that you want to federate with. Block domains to which you want to avoid unsolicited incoming communication and to prevent outgoing instant messages (IMs), audio/video calls, and file transfers. Allow domains for which you want to allow incoming and outgoing communication.