Source: https://en.wikipedia.org/wiki/Model_rocket_motor_classification

Rocket motors are classified by their total impulse, or the change in momentum the motor can impart on a system. Some further identifiers are sometimes assigned to certain classes.

Class by Impulse

The class of a motor is determined by what range its impulse falls into, as defined in the table below.

Class	Total Impulse (lbf*s)
Micro	0–0.07
1/4A	0.071–0.14
1/2A	0.141–0.28
A	0.281–0.56
B	0.561–1.12
C	1.121–2.25
D	2.251–4.5
E	4.51–8.99
F	8.991–18.0
G	18.01–36.0
H	36.01–71.9
I	71.9–144
J	144.01–288
K	288.01–576
L	576.01–1,151
M	1,151.01–2,302
N	2,302.01–4,604
O	4,604.01–9,208
P	9,210–18,400
Q	18,400–36,800
R	36,800–73,700
S	73,700–147,000

Class by Certification

Each certification level of NAR or TRA allows for a higher motor class to be used. If multiple motors are installed in a single rocket, the sum of their impulse is used. Any rockets with more than 1 motor, or a non-commercial motor are considered "complex rockets" and require Level 2 to launch no matter what.

Level	Maximum Class
Low-Power	G
Level 1	I
Level 2	L
Level 3	S*

*while the Level 3 certification allows for any sized motor, S is where most scales and insurance policies end

Launch Initiative

Rocket Motor Classification

Class by Impulse

Class by Certification