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Here is the official word from Microsoft TechNet, The C:\Users folder is shared when any sub folders from C:\Users\<username>\ is shared. The article is stale and mostly unhelpful, but worth knowing to complete the story.
A shared disk file system is one in which a number of machines (usually servers) all have access to the same external disk subsystem (usually a SAN). The file system arbitrates access to that subsystem, preventing write collisions.