The Microsoft Lync Server 2010 SDK includes library, tools, documentation, and sample applications for developing server-side SIP applications to customize and extend the functionality of Microsoft Lync Server 2010.
A good example server side application is the client filter installed by default. It inspects SIP packets and restircte certiain client versions. If you need to build an application like this you need this SDK: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=58ccf6fd-31db-4f15-bafb-c5ef28afc5fc