This is not the same as casual. Informal and Semi-formal mean the same thing.
Informal, Semi-formal or After Five Attire for Women
If it is an evening event, go with a cocktail dress or elegant separates. If it is a daytime semi-formal event, wear a knee-length dress, a dress suit or dressy separates.
Informal, Semi-formal or After Five Attire for Men
An evening event requires a dark, dressy suit with or without a vest, high quality white dress shirt, conservative tie, leather dress shoes and socks. For daytime, men can wear a light or dark color dressy suit.