There are 4 unique codex entries in the Achievement: Titles section on Korriban. They are all class specific.
Acolyte is obtained from speaking with Overseer Tremel during "Arm Yourself" mission.
Codex type: Achievement: Titles | Faction: Imperial | Class: Sith Warrior
Associated Achievement: None
Apprentice (inq) | Coordinates: -299, 66 | Location: Korriban: Sith Academy: Level 2
Apprentice (war) | Coordinates: -299, 65 | Location: Korriban: Sith Academy: Level 2
Apprentice is obtained from speaking with Lord Zash during "A Map for the Future" mission as a Sith Inquisitor player or Darth Baras during "The Final Trial" mission as a Sith Warrior player.
Codex type: Lore | Faction: Imperial | Class: Sith Inquisitor & Sith Warrior
Associated Achievement: None
Darth is obtained from speaking with Darth Baras during "Retribution" mission.
Codex type: Achievement: Titles | Faction: Imperial | Class: Sith Warrior
Associated Achievement: None
Imperial Scholar is obtained from interacting with The Red Engine during "[HEROIC 2+] The Hate Machine" mission.
Codex type: Achievement: Titles | Faction: Imperial | Class: Sith Inquisitor & Sith Warrior
Associated Achievement: None