I include a quote from Cunard's web-site pages which will probably explain all:
"Formal: Black tie or formal dark suit for gentlemen. Evening dress or other formal attire for ladies. (A range of gentleman’s formal wear is available to hire or purchase onboard ship).
Semiformal: Jacket and tie for gentlemen. Cocktail dress or trouser suit for ladies. No jeans.
Elegant Casual: Jacket, no tie required for gentlemen. Dress, skirt or trousers for ladies. No shorts or jeans."
Quite honestly it is worth going through the detail on a web-site or in the brochure as there are so many things which are worth knowing before you go and will add to your enjoyment and lack of worry on a cruise....regards...Nei.