A UNC always references a network path. You can use \\?\ to reference a network path, but it can also be used to reference a local path too. \\?\ is not always used to represent a UNC.
\\?\ is an unparsed path - the path is passed directly to the filesystem, bypassing checks and other Win32 API restrictions - a side effect of this is that maximum length checks are bypassed, but this is NOT the only effect.
\\?\ is an extended-length path.