Enables Microsoft Office Communicator to send and receive instant messages securely when using the SIP Communications Service. ...

"Enables Microsoft Office Communicator to send and receive instant messages securely when using the SIP Communications Service.  If you enable High Security, Microsoft Office Communicator requires the TLS transport. If the server authenticates users, it must use either NTLM or Kerberos authentication. Instant messages and Subscribes must pass through the SIP server, preventing outbound peer-to-peer communication.  If you enable Low Security, Microsoft Office Communicator can use any transport and any authentication method (including Basic or Digest). Instant messages can pass directly between clients, enabling outbound peer-to-peer communication.  If you enable Medium Security, all the High Security applies, except that TLS is not required.  If you do not configure this policy setting, Microsoft Office Communicator can use any transport, but if it does not use TLS and the server authenticates users, it must use either NTLM or Kerberos authentication. Instant messages must pass through the SIP server, preventing outbound peer-to-peer communication.  Note: This policy does not affect Remote Assistance.  Note: You can configure this policy setting under both Computer Configuration and User Configuration, but the policy setting under Computer Configuration takes precedence."