what can my sega cdx do?

Sega Neptune, been a long time since i seen one of those, believe it or not, last time i seen one, i played it. It was in Best Buy, I dont remember if it was for sale or display only, but i do remember it was playing doom, I didnt ask to buy one. I got a Saturn instead

I've heard rumors that very few were released (like 100) but maybe that was just a display, one that got called back to sega in a couple of days.