I do not have Domain Controller in my Configuration list, but my first thought is this: a Domain Controller is simply a Windows Server that is running special software to interact with/run Active Directory. The server is a Windows Server, and then you would have AD Domain Controller related to that Windows Server in the CMDB. This same Windows Server could also be running other software, such as SQL Server and other items that could be broken out as related items in the CMDB. In this way, the physical server as a Windows Server with related items makes sense to me.

Ben