The Mazda J platform is an automobile platform for rear wheel drive fullsize cars. It only existed for five years in the early 1990s, but was the basis for the famous Eunos Cosmo GT car.
Note that some Mazda vehicles appear to have similar platform codes in the VIN, but are unrelated. The following vehicles use a Suzuki keicar platform, not the Mazda J platform:
The JC chassis was a totally new platform developed for the 90's & is unique to the JC COsmo. Its closest cousin is the HD-929 from 91-95. It was a major update from the previous HB platform used by the former 82-89 Mazda Cosmo.