RCI has it, too. I consider smart casual to be what I call "church clothes". One night it was decided to go back to "informal" because it was country-western night at the disco and some people wore "cowboy" clothes and boots.
On Princess, "Smart Casual" really means what "Casual" was supposed to mean in eveningwear -- and if you read the "suggested" attire, you will discover that it is the same. The problem was that some passengers were ignoring the dress code and showing up for dinner and other evening events in bluejeans, "T" shirts, and other grossly inappropriate attire. They added the adjective "Smart" as a hint that such attire was not appropriate in the evening.
That said, I must say that the prescription of "Smart Causal" seems to have brought a marked improvement in appearance on casual nights along with no small amount of confusion as to what it means (don't people read their information packages???).. Not only are the jeans and "T" shirts gone, but most ladies now are wearing dresses or skirts and fancy blouses rather than slacks.