when a cat holds its straight tail up high what does it mean? alertness or curiosity?
Excitement, contentment, or happiness. Here is a good illustration:

That's not a bad diagram. I have to disagree with a couple of them, though. Sometimes the tail isn't a good indicator without the ears; the cat may have some issue (especially a housecat, because they get stepped on!) with certain tail positions.
that is true, you have to look at the whole cat, not just parts of it. For example my partner accidentally stepped on our cat's tail and it looked like the "angry" picture, however he was very loving and curious. So I guess it was just a reaction to the pain. He also does it when he is scared. But then the ears and the eyes are what I am looking at. Usually when your cat holds the tail up high while walking towards you is a sign of friendly curiosity or excitement. My cat though would whipp his tail, as he grew up with a dog in his old home. So it also depends on the cat and the character... Hard to tell all in all