so this is a topic i've wanted to make for a bit, especially with my goku still maintaining his tail. as it is more or less, an entire limb, it would make sense for it to have it's own influence when it comes to body language and even utility.
saiyan tails are just as autonomous as the rest of the body's limbs, but is much stronger. the strength of the tail can carry double the weight of an average saiyan, to the point where they can casually sit on their tails while they hold them up in the air or even hang from trees with them- as saiyan physiology is naturally strong to begin with, this is only added to the strength of the tail. especially towards those who have trained against it's weakness.
the tail is what draws out the saiyans true power in the oozaru, however it comes with the costly weakness of sapping a saiyan of all their stamina and muscle control the moment it is grabbed. with enough time and practice however, a saiyan can strengthen their tail enough that this drawback is practically non-existent, and with the mastering of this disadvantage, comes the advantage of full control of the oozaru form. many saiyan children and teenagers will practice their tail strength through various strength games and playful combat- such as grasping each other by the tail and pulling in opposite directions like a makeshift tug-o-war.
something that is actually relatively unknown to many modern saiyans is the use the tail provides when it comes to the super saiyan transformation. as the super saiyan is a transformation that is tightly knit in the saiyan biology and emotions of rage- the tail, being connected to a different rage based transformation- assists it in that regard once strengthened as well. with a tail, all super saiyan transformations: whether god based or not, receive a massive amount of reduced stamina lose and acclimation, providing a much easier time when it comes to mastering these transformations as if it were one's base form.
now for the real meat: saiyan tail body language. some of these are either learned by socialization with other tailed saiyans while some are also instinctual as connected to emotions.
wrapped around waist: neutral postion. as the tail is an obvious weakness, most saiyans have adapted to wrapping their tails around the waist like a belt as a way to protect it.
lazily hanging: feeling safe, relaxation.
raised frazzled: stress, irritation, anger
spiral curled against back: fear, stress
wagging: happy, excitement
wrapped around other's limb (non-tail): comfort, reassurance, affection, familial love
wrapped around other's tail: comfort, affection, reassurance, reserved primarily for lovers and mates
standing up straight: alert, cautious
down in between legs: submission, relinquishing control