Hi Chris
It's a modification to the internal circuitry to allow a Serial Digital Interface output on the 598 DVD player. It's an interface more commonly found in the professional video world. The interface allows an uncompressed digital stream which, when fed into an external video processor, can improve the picture quality on a monitor/plasma/projector in comparison to the usual RGB SCART or component output on the 598.
According to some commentators, the HDMI interface will make the SDI one obsolete.

Cheers

Mark

C61R, Denon DVD-A1XV, Sony CDP-228ES, UHD722, D5KC, D5500.2K, DSP33, Sony UDP-800, AppleTV 4K, Sony KE85XH9096BU