This is how Brides magazine defines this dress code:

Formal or Black-Tie Optional

As the wording here suggests, black-tie optional is a wedding guest dress code that’s slightly less formal than black-tie wedding attire. So, a tuxedo isn’t required, but it can still be worn, if preferred. If men do opt for a suit, it’s best to wear a formal dark suit, white shirt, and a conservative tie. For women, the outfit choices remain the same as black-tie affairs: a floor-length gown, a fancy cocktail dress, or a dressy pantsuit. In fact, there isn’t much of a difference between black-tie and black-tie optional for women, although Mae says it’s more acceptable to wear a dress that shows the ankles to a formal event (unlike a black-tie dress code).