Digital I/O or just O?

I’ve noticed in the back of my Audigy 4 where it says Digital IO port. I have it connected to my Sony DD 5.1 receiver for surround sound when watching movies on my second LCD screen. I now want to do more with the sound card and read Creatives specs where it says I have upgrade options for Optical Digital Interfaces for digital I/O via this port and the upgrade port on the card itself. I have looked all over and can’t seem to find this “Optional” devices (like the ones for other sound blaster cards). So, is this IO port just an O or can it receive digital signals? Is there a place where I can actually find these options? I’ve found the ones for Live! cards but nothing about other types for the Audigy’s.