I thought they defined it on their web site but maybe I'm incorrect about that. But men must wear a shirt with a collar and slacks. Women can wear slacks, capris or skirts. No denim.
My best advice for packing to Alaska (we've done 2 cruises there) is to bring long sleeved tee shirts, a sweater or sweatshirt or two and a lightweight waterproof jacket with a hood. Dress in layers and bring a backpack on excursions so you've got a place to put your extra layers.
Weather can change very quickly. I remember cruising the glaciers and being a popsicle, bundled up with only my nose sticking out and 30 minutes later as we got closer to the glacier being down to just my tee shirt and gloves. Well, okay, I did still have my jeans on.
It's a magnificent trip and being able to dine freestyle makes it that much nicer. There are times when you are cruising an area right at supper time and it's so nice to know you're not missing out on either your dining experience or glorious scenery. Enjoy!