Gentleman's dress - Formal or Semi-formal

According to the Formal dress code, men wear classic suits, preferably black or evening blue.

In some social circles Semi-formal can also mean Black Tie. But in the general case this is a term for the most informal of formal dress standards. Instead of belt, a vest and a bow tie in another color can be added to the tuxedo. A colored vest is also permissible. However, gentlemen most often appear in classic suits and Oxford shoes in these cases.

Richmart Semi-formal men's suit

As a rule, Semi-Formal suggests white shirts with 'sandwich' cuffs and cufflinks, but a black shirt with a bow tie is also possible.

See also:
Gentleman's dress - Apotheosis of tailors
Gentleman's dress - White Tie
Gentleman's dress - Black Tie