Tail up means either aggression or dominance or ready to mate.
Usually with full aggression, while the tail is up, the ears are flat back (ready to battle and get the ears out of the way), usually accompanied by a quick movement forward or lunge in the other’s direction.
Dominance is similar, but without the lunge. They can greet each other, but the one with the tail up is saying – I’m meeting you as a dominant and you’d better pay attention or there will be hell to pay. I may even spray you to prove my point – the tail is aimed and ready to go.
Mating – Jack always puts his tail up and circles when he wants to mate with Rucy. He honks his loves honks and he can also spray to “claiim” his female. He doesn’t seem to get it that he’s neutered and that Rucy is the only female around and there has never been another male around all these years – doesn’t matter.
Hope that helps.
Can you refresh my memory – have they met yet face to face?