This specific update generally includes various balance tweaks and bug fixes implemented by the developers, Bulwark Studios, after the initial launch.
The Warhammer 40k Wiki defines a "Heretek" as a Tech-priest who defies the Cult Mechanicus to pursue forbidden technologies. In the context of the game, this expansion introduces: Warhammer.40000.Mechanicus.Heretek.v1.4.0-CODEX...
The "CODEX" tag at the end indicates that this is a release from the digital piracy group known as CODEX , who were known for "cracking" game digital rights management (DRM) before they disbanded in early 2022. Warhammer.40000.Mechanicus.Heretek.v1.4.0-CODEX...