The "Codex" in the context of Assassin’s Creed Rogue often refers to the (specifically the "Collector’s Case") which featured a physical artbook and a stylized box. However, in the digital space, "Codex" refers to the hidden layers of Templar and Precursor history that Shay uncovers throughout the game.
The Forgotten Chapter: Unlocking the Assassin’s Creed Rogue Codex Exclusive Content assassinscreedroguecodex exclusive
The tag generally applies to the additional missions and customization items that were not included in the standard base game at launch. The "Codex" in the context of Assassin’s Creed