To be more specific:
M = engine series
A = chassis series (RWD Celica/Carina/Supra)
6 = chassis generation (ie 6th generation of the A chassis)
7 = model variation
To confuse matters, in some cases the engine series may be two letters, and the chassis generation may be two numbers. For example, the current Celica is designated ZZT230:
ZZ = engine series
T = chassis series (FWD Celica/Carina)
23 = chassis generation
0 = model variation
To add more confusion, here is the rundown on ALL of the known chassis codes for all of the Supra's that were ever built,
MA45: MKI Supra/CelicaXX w/ the M-EU 2.0L motor.
MA46: MKI Supra/CelicaXX w/ the 4M-E 2.6L motor.
MA47: MKI Supra/CelicaXX w/ the 5M-E 2.8L motor.
MA61/MA67: MKII Supra/CelicaXX w/ the 5M-GE DOHC 2.8L motor.
GA60: MKII Supra/CelicaXX w/ the 1G-GEU 2.0L motor.
MA63: MKII Supra/CelicaXX w/ the M-TEU 2.0L motor.
MA6?: MKII Supra/CelicaXX w/ the 5M-E SOHC 2.8L motor. (Does anyone know this chassis code??).
MA70: MKIII Supra w/ the 7M-GE 3.0L motor.
MA71: MKIII Supra w/ the 7M-GTE 3.0L motor (this chassis code DOES NOT exist in Japan, both the 7M-GE & 7M-GTE uses the MA70 chassis code. The MA71 is basically a US reference)
GA70: MKIII Supra w/ the 1G-GTEU 2.0L motor.
JZA70: MKIII Supra w/ the 1JZ-GTE 2.5L motor.
To my knowledge, both the 2JZ-GE & the 2JZ-GTE MKIV Supra's use the JZA80 chassis code. Corrections????
I might add that the MA71 did exist in Japan; a friend of mine purchased a JDM front cut which was an MA71. The MA71 certainly isn't a "US reference" because some Australian-delivered models were also MA71's. To add even more confusion, I have seen MA70's with factory-fitted 7M-GTE's.