If you need to track call detail records (for example, if you do want to collect statistical usage data without archiving the actual body of the IM conversation) or Quality of Experience (QoE) data, you should deploy the Monitoring Server.

To simplify data monitoring, Office Communications Server 2007 R2 consolidates the Call Detail Record (CDR) service with QoE data collection into a single server role, called the Monitoring Server. Because the CDR service is no longer hosted by the Archiving Server, organizations that require the CDR service are not required to deploy the Archiving Server. The Office Communications Server 2007 QoE Monitoring Server is deprecated in Office Communications Server 2007 R2.