I don't know the bike, but 63 - 66 is usually right for 700c and 66-69 tends to be for 26" wheels. I assume you're looking at a derailleur that might be used on a MTB or trekking bike? Most road orientated stuff doesn't have the option.
I didn't know there was any difference and transferred a groupset from a MTB to 700c hybrid, it never shifted smoothly, but always shifted. A LBS couldn't work it out, but a club rider knew, swapped and it made the difference. Apparently Dawes supplied a model with the wrong one for years and it worked, though maybe not as well as it should. so, getting it right matters, but it isn't fatal in you don't.
