the A just refers to a basic type of platform. In this case the A is shared between Celica, Carina and later the Supra and Soarer. It does not have any relation to FWD or RWD. The 2 refers to the generations, the floorpan is not always carried forward to the next generation so parts do not always interchange.
The Celica actually changed code from A to T around 1985 when it became FWD. The A code carried on with the Supra and Soarer.
There are some very informative sites on chassis codes if you use Google.
If I've made any errors I'm sure someone will correct me.