Chassis codes are as follows (according to Japanese models):

1st letter determines model type:
G = Impreza
B = Legacy
S = Forester

2nd letter determines body type:
C = Sedan 1st gen. (1993-2000 Impreza, 1989-1994 Legacy) + Coupe (Impreza)
D = Sedan 2nd gen. (2001-current Impreza, 1995-1999 Legacy)
E = Sedan 3rd gen. (2000-2003 Legacy)
L = Sedan 4th gen. (2004-current Legacy)
F = Wagon 1st gen. (1993-2000 Impreza, 1998-2001 Forester)
G = Wagon 2nd gen. (2001-current Impreza, 1995-1999 Legacy, 2002-current Forester)
H = Wagon 3rd gen. (2000-2003 Legacy)
P = Wagon 4th gen. (2004-current Legacy)

3rd letter determines engine type:
A = 2nd gen. Impreza WRX w/ 2.0L turbo
B = 2nd gen. Impreza WRX STi w/ 2.0L turbo high output (we also use this to refer to 2.5L STi)
8 = 1st gen. Impreza WRX w/ 2.0L turbo (includes STi)
5 = Forester & Legacy 2.0L turbo (we also use this to refer to US 2.5L turbo Forester/Legacy)

4th letter determines body revision (generally changes each model year):
A = 1st year
B = 2nd year
C = 3rd year
etc. (for GC8 models C = both 1995 & 1996 model years)


An example would be:
"GDB A/B/C" = Impreza Sedan STi model 2nd gen. (New Age) model year 2001, 2002, 2003 (in US
this would also apply to 2004 Impreza STi because 2003 japanese model year is equivalent to 2004)

A second example would be:
"GD & GG" = applies to all 2nd gen. (New Age) Impreza models regardless of body or engine type

Engine codes are as follows:

EJ20G = 2.0L turbo WRX engine used in early GC8 models
EJ20K = 2.0L turbo WRX STi engine used in Version III & IV models
EJ205 = 2.0L turbo WRX engine used in all 1999 to current WRX models
EJ207 = 2.0L turbo WRX STi engine used in all 1999 to current STi models
EJ255 = 2.5L turbo engine used in US Forester XT & Legacy 2.5L turbo models (+ some overseas)
EJ257 = 2.5L turbo engine used in US Impreza STi models