From Sega Retro
The Sega Card or Sega My Card media format was an alternative to cartridges, designed by Sega for use with the SG-1000 II, Sega Mark III and Sega Master System. The latter two used cards with the name Sega My Card Mark III in Japan, to differentiate them from those designed for Mark II hardware.
Sega Cards were typically cheaper to produce, but have limited storage capacity. The idea at the time was to distribute bigger games on cartridge to be sold at a higher price, while smaller games would be distributed on card at a reduced price. Sega backed both formats during the 1980s, but ultimately scrapped the Sega Card format as third-party developers preferred the cartridge format. The majority of games distributed on card were also distributed on cartridge, making the format obsolete in western regions very early on.
Sega Cards are not region locked, unlike Master System cartridges which are of a completely different shape in non-Asian regions. Systems without a card reader (the original SG-1000, and redesigned Sega Master System II) can use the Card Catcher to run card-based games. The Power Base Converter was the last piece of hardware to contain a card slot, however, as the Sega Mega Drive cannot run SG-1000 games, the slot is restricted solely to Mark III/Master System cards.
The Sega AI Computer also ran Sega cards, though they were incompatible with other systems.