For example my idea of casual is a T-Shirt and boots where as someone of a not so recent generation’s idea of casual might be a shirt with no tie.
In standard vernacular (and in certain circles), "casual" could very well mean a t-shirt. But the term "Highland casual" probably doesn't mean that, since (I'm guessing) t-shirts are not really as common there as they are in the USA. I could be wrong.

Invoking the term "Highland casual" probably refers to a more traditional sense of casual that's steeped in Highland history, and so that should probably be taken in context. Not knowing anything else about the organization or the people, I'd be inclined to wear a kilt with traditional hose and flashes/garters, comfortable dress shoes, and a buttoned/collared shirt at the very minimum. But since it's an evening event with a possible concert, I'd go with a tie and waistcoat as well... possibly a jacket too. Sort of the "day wear" look, even though it's an evening casual event (but with black shoes and belt).

That's actually a good question for the experts. In relation to the "10 looks" thread, would "evening casual" be perhaps somewhere around "smart day wear"? Or would it step up to "semi formal (suit equivalent)"?