There are 9 unique codex entries in the Achievement: Titles section on the Imperial Fleet. 1 is accessible to all players regardless of class or faction and 8 are unobtainable.
Red Rhombus is obtained from speaking with Colonel Jadick during "Terror from Beyond" mission.
Codex type: Achievement: Titles | Faction: Both | Class: All
Associated Achievement: None
The Assassin is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Sith Inquisitor
Associated Achievement: None
The Juggernaut is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Sith Warrior
Associated Achievement: None
The Marauder is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Sith Warrior
Associated Achievement: None
The Mercenary is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Bounty Hunter
Associated Achievement: None
The Operative is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Imperial Agent
Associated Achievement: None
The Powertech is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Bounty Hunter
Associated Achievement: None
The Sniper is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Imperial Agent
Associated Achievement: None
The Sorcerer is unobtainable.
Codex type: Achievement: Titles | Faction: Imperial | Class: Sith Inquisitor
Associated Achievement: None