Champion numbering uses a letter to designate size and reach of thread, a number to indicate heat range and another letter to indicate gap type. Heat range numbers go up as heat range goes up. Resistor plugs have "R" as a PREFIX to the normal numbering.
Here's a brief breakdown *in general* :
A or R = 12mm thread, .750" reach, 11/16" hex
BN = 14mm thread, .708" reach, 5/8" hex
BL = 14mmThread, .460 reach, 5.8"hex, conical seat
D = 18mm thread, .455 reach, 7/8" hex
K = 18mm thread, .445 reach, 1" hex
F = 18mm thread, .460 reach, 13/16" hex
G = 10mm thread, .750 reach, 5/8" hex
J = 14mm thread, 3/8" reach, 13/16" hex
L = 14mm thread, .500 reach, 13/16" hex
N = 14mm thread, .750 reach, 13/16" hex
Normal plugs have (usually) a one-digit heat range number (there are exceptions) and "Racing" plugs have a two-digit heat range number (there are exceptions)
Suffixes denote the gap type:
no suffix = normal gap/electrode(s)
Y = projected nose
G = gold palladium center electrode
R = retracted gap
V = surface gap