Specify CIFS vs NFS for UNC in Windows?

On Windows 2008 Server R2 (acting as a client) I have the Client from Windows Services for NFS installed and a serve开发者_如何学Pythonr mounted as Y: with NFS. That server also supports CIFS. When I open a UNC path to the server with start/run it connects using CIFS but when an application on the same client is passed the same UNC path it connects using NFS. The full UNC path (not the Y: drive letter) is used in both cases.

How does Windows determine whether to use NFS or CIFS when passed a UNC path?