Drive vs Drive2

Our 61 Mercury had the D1 and D2. The dealer tried to explain it and not sure if it was a sender or receiver problem, but I always thought D2 was like an overdrive. I never could tell much difference in the two and the dealer said drive in either one, didn’t matter. Starting in second gear I guess would make more sense but can’t really see going to all that effort to provide a way to start in 2nd. Maybe but I know Ford had overdrive in their manual transmissions as early as 1954 so just figured they found a way to put it in the automatics. Of course I was a little young to drive it much.