This dress code in high society is considered informal.

Informal may sound casual and it is compared to White Tie and Black Tie, but it normally refers to a respectful standard of dress. Typically Lounge Suit and Cocktail Dress are worn to such events.

In very relaxed events it might mean Smart Casual, but generally, unless you know for sure it is best to stick to the dress code, to present a sophisticated image to new people you may meet. It's always better to be overdressed than underdressed.