Don't see the confusion, we just follow what the cruise line requests, for instance this is what P&O say...
This is the opportunity for ladies to wear formal evening wear such as an evening dress, smart trouser suit or a cocktail dress. For men, a dinner jacket or tuxedo are the norm, otherwise a dark lounge/ business suit and tie or formal national dress is fine. (Applies to all ships.)
Ladies can wear anything from tailored trousers to smart separates or an elegant dress, while men are required to wear a smart shirt and trousers with a jacket such as a blazer or sports jacket. A suit and tie may be worn but is not compulsory. (Semi-Formal applies to Arcadia, Artemis, Aurora, Azura and Oriana only.)
Stylish resort or leisurewear is ideal, for example casual separates or dresses for ladies and open-neck polo shirts and casual trousers (not shorts) for men. A jacket and smart trousers may be worn but are not compulsory. Smart dark denim may be worn however we request that the following items are not worn: sportswear, e.g. trainers, football shirts and tracksuits. Bring your brightest clothes for our Tropical evenings. (Applies to all ships.)