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.
According to Oscar Wilde who was very conceited, the knot of tie of the gentleman speaks about him more than secular gossips can tell.