"TRM" which is short for trim. It is the interior codes for the trim, seats, and color. This is a brief list, many more are used.

Under T (this is the trim grade)
C - Charger
H - High
D - Deluxe
P - Premium
S - Sport

Under R (this is the seat code)
4 - vinyl split bench seats
6 - vinyl bucket seats
7 - vinyl & cloth bucket seats

Under M (this is the interior colors)
A - gray
B - light blue
C - white & blue
D - white & green
E - white & gold
F - green
H - white & tan
L - tan
N - gold & black
Q - turquoise
R - red
S - silver & black
V - white & red
W - white & black
X - black
Y - gold