You are correct, those are times to maturity. Some of the long day onions will mature in as few as 75 days. They are called long day, because they need 14 - 16 hours of daylight to bulb. Short day onions have a long season 115-135 days but began bulbing with 10-12 hours of daylight. These are normally grown in the winter for spring harvest.
You should be able to grow any long day onion with a DTM under 100. Probably even longer because onions are not overly affected by frost.

But how do I know which ones are long-day onions? This was my question. The catalogue makes no mention of day length, only season. So, you're saying that everything in the early and perhaps second early sections are long-day and everything in the late section is short-day? Where does mid-season fall �" long or short day? How would I know which (if any) are day length neutral?

I looked through Stokes 2010 list, all are long day onions. There are not a lot of varieties that are day neutral. Candy, Red Candy Apple, and Super Star are the most readily available. There are about 3 dozen varieties of short day onion, but they are suitable only for the deep south. The Spanish onions are long day but have a long season, they do well in the Pacific northwest, but may have too long a season for you.

Awesome, farmerdilla... THANK YOU so much! Now I can forget about that aspect altogether and just pick what suits my season length. :)

Did you just recognize the varieties or was there information in the online listings that I didn't see?

I am in a short day onion growing region, so I recognise most of the short day varieties, As I noted there are not many intermediate/day neutral varieties so they are pretty easy to track. Stokes does not cater to customers in the southeast or southern California so no need to carry short day varieties. Most of the seed companies that do carry them will clearly identify them.