O.K. we need to narrow down which models we are talking about first, Garmin make a great many Devices with a GPS inside and the letters and numbers won't always mean the same, there are basicslly 3 parts to the name so in your example above we have (1) = Drive, (2) = 61, (3) = LMT-S
The first part usually describes where the unit will be used, in your case it's on the road, but it could also be for walking / hiking, flying, marine, cycling, running etc..
Now we are focusing only on 'on-the-road' devices we have a smaller list for (1) above, they include :-
Streetpilot, Nuvi, Zumo,Dezl, RV, Fleet, Drive, DriveAssist, DriveLuxe, Drivesmart etc.
The (2) section is usually a number of 2, 3 or 4 digits (very rarely a single digit) where the first digit is often the screen size in inches and the other digits indicating smaller physical differencies e.g. Bluetooth etc.
The (3) section often shows options that can be applied the same physical unit and includes different software, maps and other 'soft' changes or hardware add-ons, here are the ones I know of :-
L = Lifetime ( usually free map updates of Traffic info.)
M = Map updates (3 - 4 times a year)
T = Traffic info. (Garmin Traffic)
T - D = Traffic info. (Garmin Digtal Traffic)
THD - Traffic info. (Garmin HD Digital Traffic)
T - S = Traffic info. ( Live Traffic info, via a smart link e.g. Bluetooth)
NOTE :- Not all of combinations of Numbers (2) and Letters (3) are available, e.g. I don't think Drive 6 M exists
