The vast,vast majority of people today know "Formal" as suit and tie.
If they mean Tux, they either write "smoking/tuxedo" or "black tie". And even then, the later can also just mean "Dark suit and black tie or bowtie", I've experienced this countless times. Even showed up in Tux to a "Black tie"...turned out to be the only one in a Tux that night.
How very true.