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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.
It is a device or piece of information on a computer that can be remotely accessed from another computer, typically via a local area network or an enterprise intranet, transparently as if it were a resource in the local machine. Network sharing is made possible by inter-process communication over the network. Some examples of shareable resources are computer programs, data, storage devices, and printers. E.g. shared file access (also known as disk sharing and folder sharing), shared printer access (printer sharing), shared scanner access, etc. The shared resource is called a shared disk (also known as mounted disk), shared drive volume, shared folder, shared file, shared document, shared printer or shared scanner.