In modern times, we have terms like “smart casual” and “cocktail attire” which are so subjective as to be meaningless.

Historically, terms like “formal” (white tie in the U.S.), “semi-formal” (black tie in the U.S.), and casual (meaning suits) had specific, objective meanings.

I, too, have seen some guidance on the internet saying diced hose should only be worn by veterans. However, I do not agree. That seems to be merely the opinion of random internet sources. Historical and reliable contemporary authorities all suggest diced or tartan hose as correct and appropriate for formal and semi-formal evening attire.