The CD-ROM² System add-on for the PC Engine was NEC's attempt at extending its home console to support CD-based media, similar to the Sega Mega CD's relationship to the Sega Mega Drive. In North America this unit was released as the TurboGrafx-CD - no Sega games were released for the system in this region, so Sega Retro uses the Japanese model and naming scheme. In Japan, NEC Avenue released half a dozen ports of Sega arcade games for the system.
There are several types of CD-ROM² games, dictated by the amount of RAM the PC Engine system in use had. CD-ROM² games simply use the resources of a standard PC Engine. Super CD-ROM² (SCD) games require more RAM, and Arcade CD-ROM² games (ACD) need even more. No Sega games were released on ACD format, though a couple used SCD. Japanese consumers could purchase special HuCards which added RAM to the setup, allowing for the more advanced games to be played. Sega did something similar with the Sega Saturn, and its 1MB and 4MB RAM Extended RAM Cartridges.
Both CD-ROM² and Super CD-ROM² games are contained within this category. As there are only a few games in total, it is not worth making the distinction.