I know it is probably NOT in the cruise contract, (although the cruise contract states that we have to follow all rules and regulations) but here is what Princess writes in "Before you go".
See highlighted part in RED.
We most of the time ditch the formal wear in favour of dining in either the buffet or one of the specialty restaurants.
PACKING FOR A PRINCESS CRUISE VACATION
While planning a trip to the Caribbean or Europe with Princess Cruises, many passengers are faced with the daunting task of packing the perfect amount of clothes and travel necessities for their globe-trotting adventure. On many world cruises, passengers visit numerous continents with varying climates, whether taking a shore excursion in the cooler temperatures of Helsinki, Finland, or lounging on the beach in the Mediterranean. From bikinis to ball gowns, these packing essentials are perfect for any vacation on a Princess cruise.
Before opting for vacuum-sealed bags and travel-sized toothbrushes, cruisers may want to thoroughly research their cruise destination as well as the activities available on board. Princess Cruises offers a number of formal dining nights during which ties and coats are mandatory. While some ships provide rental services, male travelers may want to add suit jackets and slacks to their packing list. Women, on the other hand, should bring along dresses made with wrinkle-free fabric, which will survive long hours stuffed in a suitcase. Passengers need not worry about having too few clothes, however, because each ship is outfitted with onboard laundry service.
While embarking on an Asian cruise, female passengers will want to avoid wearing shorts and skirts above their knees due to the conservative fashions worn by many Southeast Asian locals. Additionally, many Caribbean golf courses have a strict dress code, particularly in Bermuda, so travelers thinking of hitting a few rounds should come prepared with collared shirts and clean shorts. Many European nations do not have a stigma against casual fashion, but locals often sport chic clothes. Before leaving the cruise ship for a European shore excursion, Princess passengers may want to change out of their sweatpants and flip-flops.
Other than fashion concerns, vacation packers cannot forget to bring the essentials. Because cruise trips are usually at least one week long, passengers should bring extra camera batteries and memory cards as well as a power strip to charge several electronics at once. If falling asleep in moving modes of transportation is difficult, some Princess passengers may want to bring earplugs, eye masks and sleeping aides to make travel more enjoyable. Over-the-counter motion sickness pills are vital, and while on the upper decks, vacationers should have ginger snacks on-hand to soothe their stomachs.
Notorious under packers do not have to worry about forgetting items because many travel essentials, including clothing, are available at ports and in the boutiques on board Princess cruise ships.