The Protocol Engineering Core Unit is responsible for extending the functionality of the Maker protocol, assisting with the maintenance and operation of existing smart contracts, and ensuring the safety and correctness of protocol design and implementation. See their mandate here.
Membership of Protocol Engineering can be seen in the @Protocol-Engineering group.
A Core Unit is a building block for organizing work for the DAO. Facilitators define long-term work areas and cover a broad set of focus and responsibilities for the protocol to be secure and successful. Core Unit Mandates set the direction of the work that Facilitators must help carry out. To read more about Core Units, head to MIP39: Core Unit Framework.