It is a friendly greeting with some apprehension. Of our seven cats, one is submissive and often has his tail between his legs (though as he's gained confidence, more and more often it's just straight up). ..the flip side of Billy (the submissive kitty) is that he almost always greets us with a quivering tail.
The others all do the ramrod straight tail - except for Tuxedo, who almost always has the question-mark tail. He is more - edgy - than all the other kitties...always more "on alert" than the others (also more aggressive
This is the short version - there is text to go with it.