How does Calibre handle ePub 3s?

So far, using Calibre 2.68 (macOS) it seems like whenever an ePub 3 is added to the library, Calibre simply parses the ePub 3 metadata and outputs an ePub 2. I haven't been able to get it to output an ePub 3 (with ePub 3 formatted metadata, using <meta property=...>).

Am I doing something wrong? How is ePub 3 implemented in Calibre? Lastly - is there any documentation available online on the metadata fields Calibre used and their values?

Thanks!