It's regarded as polite to use 2nd for yourself if speaking to a higher-ranking Sith, or 1st for yourself if speaking to a higher-ranking non-Sith Imperial.
Traditionalist Tsis 3rd all Tsis regardless of Force-sensitivity or Jedi-Sith affiliation. Assimilationist Tsis do not.
Masters like Baras expect their apprentices to use 1st for themself, though, and masters like Zash expect their apprentices to use 3rd for themself.
(Any particularly interesting Imperial-Imperial dynamic will seesaw between pronouns a lot.)
Vette uses 2nd pronouns for herself when Irati is using 1st as Baras' apprentice. Quinn has no idea how to pronoun himself to reflect that, because he'd normally go with 2nd to Vette and 1st to Irati but it is impossible to pronoun himself lower than Irati, or as higher than Vette without suggesting he's two levels superior to Irati (besides, he'd never Sith-pronoun himself). He vacillates between 1st and 2nd until Irati starts using 2nd for herself and then sticks with 2nd. She takes 3rd in public but 2nd in private.
This is a conceit in-universe. As most of Reconstruction is written in English, and Republic Standard and Imperial Standard are not functional conlangs, the reader must use imagination unless otherwise specified.