Many of the file processing actions in Visual Build call Windows APIs to do their work.  Normally, these APIs are limited to paths up to 248 characters and filenames up to 260 characters, but they support paths and filenames up to 32,767 characters in length (extended-length paths) if the path or filename is prefixed with \\?\ (i.e., \\?\C:\Windows or \\?\UNC\Server\Share\path).  The file processing actions in Visual Build automatically add this prefix to extended-length paths and filenames when calling Windows file and path APIs.