Unlike OCS 2007 and OCS 2007 R2, Lync Server allows the A/V Conferencing Server functionality to be separated from the Front End Server role. This new server role is referred to as the A/V Conferencing Server. By separating this role from the Front End Server, one is able to noticeably improve scalability and performance for A/V conferencing.
As a rule of thumb, it is a good idea to separate off this role for sites with more than 10,000 users and to form an A/V Conferencing pool.