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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.
The UNC defines the path to a shared folder using the format \\server\sharename. The server is the computer where the shared folder is stored, and the share name is the name given to the shared folder when the share was set up. Another way to connect to shared network resources is by using the My Network Places window.