Which action should a Citrix Administrator take when using Provisioning Services to provision additional Windows Server 2012 server OS machines in a machine catalog?

Creating a new vDisk is only required during initial PVS setup; when adding machines to an existing catalog, the vDisk already exists and must be reused rather than duplicated.

Adding an OS to the vDisk store is not a valid PVS operation for this scenario - the vDisk store is a repository of existing vDisk files, and adding new target devices does not require modifying it.

Creating a new site in Provisioning Services is an organizational administrative action for separating PVS environments and is not required or appropriate when simply adding more target devices to an existing deployment.

In Provisioning Services, a device collection groups target devices and associates them with a vDisk for OS streaming - adding new virtual machine records to the existing device collection assigns them to the correct vDisk without requiring a new image. Once added to the device collection, the new machines can boot from the streamed vDisk and be registered with the XenDesktop Delivery Controller. This is the standard workflow for scaling out an existing PVS-based catalog because the vDisk and site infrastructure already exist.