Elaborating on what andrew loftus has said.
The Ace3DS Pro supports one 3DS game (don't confuse it with a ROM) and can play DS ROMs normally.
You can switch between the 3DS game and firmware for 3DS ROMs by using a little switch.
Just note this: The card has actually not being released yet. I have attempted to contact the Ace3DS Team about this, but they haven't replied to any of my emails.