E = Mult Dwelling.
J = Movie Theater or otherwise large gathering of people for a show.
If you're looking for all the letter codes for alarms, here they are:
A = Factory fire
B = Commercial Store fire
C = Commercial (other) fire
D = Private Dwelling fire
E = Multiple Dwelling fire
F = Hotel/Motel/SRO fire
G = Church fire
H = Hospital fire
I = School fire
J = Movie Theater/Other Theater/Studio fire
K = Public (other) fire
L = Building under construction/renovation
M = Vacant structure fire
N = Brush fire
O = Outside rubbish fire
P = Transportation fire
Q = Non-structural (other) fire
R = Emergency
S = Malicious False Alarm
T = Transit/railroad incident
U = Administrative Assignment
V = EMS or EMS911 CFR Incident
Each one has its own matrix response.
With central station alarms, the letter (E/J) does not conform to the Starfire letter codes. Any commercial building over 75 feet in height is required by Local Law 5 of 1973 to have a monitored alarm system. That class of building is "E" hence the Class E alarm. A Class J alarm is the same as a Class E except that the building is used as a hotel.
Class E and J Alarms, depending on location in Manhattan, get either an engine or a truck and a battalion chief. After 1900, Class J's get 3X2 w/a BC.
Outside of Manhattan, Class E and J get 3X2 and a BC.