They were secondary games for the master system but very similar to the primary ones for the TG-16.
The HuCard format is probably the first and most impressive feature of the PC Engine system. How cool was it, back then, to go and visit some friends with a dozen games fitting in the palm of your hand ? But Games on cards were not only available for the PC Engine system, far from it. Let see what else we can find...
As mentionned, the MSX also had a few games stored on cards. The Master System also had few games on cards. They were released in Japan around 1985 and were called Sega Mycard. A few of them were released in Europe and America as Sega cards. Never wondered what the slim port of the original Master System console was for ? Here you go, this was to play the tinny Sega cards.
For some pics of the sega ones it's about 2/3 down the page.