Quote from CIP-1694:
Hard-Fork initiation on “Hard fork initiation”:
Regardless of any governance mechanism, SPO participation is needed for any hard fork since they must upgrade their node software. For this reason, we make their cooperation explicit in the hard fork initiation governance action, by always requiring their vote. The constitutional committee also votes, signaling the constitutionality of a hard fork. The DReps also vote, to represent the will of every stake holder.