It's just the model number, that the japanese tend to use to refer to the car. Or that drift culture use to refer to cars.
FD = the RX-7 from 1993-2002 or whatever.
Think R32 for early nineties Nisaan Skyline etc Next model was R33, then R34...
Also, like with BMWs, E46 etc.
Don't confuse the whole FD/FC thing with FR or MR or FF lol. They're Front Engine Rear wheel drive, Mid engine Rear wheel driver and Front engine Front wheel driver respectively.
Welcome to Gran Turismo lol.