Box 3 on the FAA Flight Plan Form asks for the aircraft type and equipment suffix. The suffix gives ATC some pertinent information about your plane's equipment and capabilities.
Special Equipment Suffix:
X: No transponder
T: Transponder with no Mode C
U: Transponder with Mode C
D: DME, no transponder
B: DME, Transponder with no Mode C
A: DME, Transponder with Mode C
Y: RNAV, no transponder
C: RNAV, transponder with Mode C
I: RNAV, transponder with Mode C
G: GPS with en route and terminal capability
For example, a Cessna 310 with DME and Mode C transponder would be: C310/A
AOPA Flight Planner Form