Black tie means tuxedo and it is only optional if it says “black tie optional.” Otherwise men have two choices: wear a tuxedo or politely decline the invitation. To wear anything short of a tuxedo for an affair that says “black tie” is extremely rude and disrespectful to your hostess.

When it says “black tie optional,” it means that they want you to wear one, but aren’t insisting if doing so would be a great expense or inconvenience If you own one or can rent one without stretching too much, you should. If you can’t, come anyway but dress-up as much as you can.

Semi-formal traditionally means black tie, (although your host may have gotten bad advice about that) in which case “formal’ means white tie. (The Oscars are black tie, not white tie, by the way.) More recently, people have come to refer to black tie as “formal” and white tie as “most formal.” (Not sure where “ultra formal” comes from. I’ve never seen it on an invitation.)

As the author says, if it says “formal” or “semi-formal” these days, you’ld better check with the host what that means to them. But if it says “black tie” or “white tie” (or even “most formal”) then the meaning is clear and you can’t get away with a dark suit or a shirt that isn’t white. Unless you want to be a bad and rude guest that otehr people are snarking about.