There are 5 unique codex entries in the Game Rules section on Hutta. They are all class specific.
Bounty Hunter is obtained from interacting with Zinny during "Tools of the Trade" mission.
Codex type: Game Rules | Faction: Imperial | Class: Bounty Hunter
Associated Achievement: None
Energy is obtained from speaking with Suite Holoterminal during "Economies of Scale" mission.
Codex type: Game Rules | Faction: Imperial | Class: Imperial Agent
Associated Achievement: None
Heat is obtained from speaking with Nem'ro the Hutt during "Settling Accounts" mission.
Codex type: Game Rules | Faction: Imperial | Class: Bounty Hunter
Associated Achievement: None
Imperial Agent is obtained from interacting with Lycus Mattle during "Friends in Common" mission.
Codex type: Game Rules | Faction: Imperial | Class: Imperial Agent
Associated Achievement: None
Item Modifications is obtained from from interacting a Clickable Plaque with during "Item Modifications" mission.
Codex type: Game Rules | Faction: Both | Class: All
Associated Achievement: None