There are 3 unique codex entries in the Game Rules section on Tython. 1 is accessible to all players regardless of class or faction and 2 are class specific.
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
Jedi Consular is obtained from speaking with Master Wettle during "Wisdom of Elders" mission.
Codex type: Game Rules | Faction: Republic | Class: Jedi Consular
Associated Achievement: None
Jedi Knight is obtained from speaking with Master Muheeda during "The Arm that Holds the Saber" mission.
Codex type: Game Rules | Faction: Republic | Class: Jedi Knight
Associated Achievement: None