A grey-colored CD attachment system add-on for the PC Engine, with built-in SystemCard 3.0 to play all Super CD-ROM² games in addition to CD-ROM² game formats. It can be connected directly to the rear pinouts of the original white PC Engine, the PC Engine CoreGrafx, CoreGrafx2, and lastly the SuperGrafx. For the SuperGrafx, this is a much more efficient way to add CD-Rom expansion to it compared to the RAU-30 method. A special adaptor, named PI-AD18, is required to connect it to the PC-Engine LT.
The following diagram comes from pcenginefx.com