Formal Attire: A little less formal than black tie, but still elegant and sophisticated. Pops of color welcome, tux not required. Women in floor length gowns with elegant jewelry and refined clutches.

https://www.theknot.com/content/formal-wedding-guest-dresses

Black Tie: A black tie outfit is built around a dinner jacket, a dress shirt, formal trousers, a bow tie, and polished dress shoes. For women, we can generally assume that “black tie” equals “long.” But that doesn’t mean just any long dress fits the bill. Make sure your dress is rendered in an evening-appropriate fabric: think velvet, chiffon, silk, or lace.

https://www.marthastewart.com/7912329/what-it-really-means-to-plan-black-tie-wedding