A Soe'feia is a Truthspeaker to the Seanchan royal family, required by duty to tell the absolute truth and free from any possible repercussions and can also be asked to administer punishments (WH, Ch. 14). Their freedom to speak as they wish can often be shocking to commoners or even those of the lower Blood (WH, Ch. 14). It is considered abominable to not listen to one's truthspeaker. Members of the Blood often call their Voices of the Blood "Soe'feia", but they are so'jhin and know they can be punished if they displease their owners (WH, Ch. 14). Even Suroth, who is of the High Blood is astonished when Tuon does not punish Anath after Anath tells her she should be strapped (WH, Ch. 18).
Semirhage murdered Tuon's previous Soe'feia Neferi, so that she could take her place close to her, masquerading as Anath. Anath has been Tuon's truthspeaker for less than two years and she isn't entirely comfortable with her, even a little afraid of her (TPoD, Ch. 14),
"'I suppose this idiocy is over having the damane caned. You are a fool to think your eyes are downcast by a little thing like that. What did she say to make you angry? No one seems to know, except that you threw a tantrum I am sorry to have missed.'" (Anath to Tuon, Winter's Heart, Chapter 14).
"Many of the Blood called their Voices Soe'feia, but Voices of the Blood were so'jhin, and knew they could be punished if their owners were displeased by what they said even if they were called Soe'feia. A Speaker of Truth could not be commanded or coerced or punished in any way. A Truthspeaker was required to tell the stark truth whether or not you wanted to hear it, and to make sure that you heard. Those Blood who called their Voices Soe'feia thought that Algwyn, the last man to sit on the Crystal Throne, almost a thousand years ago, had been insane because he let his Soe'feia live and continue in her post after she slapped his face before the entire court." (Winter's Heart, Chapter 14).
"'This is the worst sort of stupidity, girl, a taste of the strap will set you right, and clear your head of nonsense.'" (Anath to Tuon, Winter's Heart, Chapter 18).