Formal authorisation establishes the legal basis permitting the activity, while the Rules of Engagement translates that authorisation into detailed, practical operational boundaries - together providing essential protection for the individual testers carrying out the work, for the client whose systems and data are involved, and for the overall integrity and credibility of the engagement as a genuine, well-governed professional exercise. This is substantive governance, not mere box-ticking (D); the underlying legal and risk-management rationale applies to any properly conducted engagement carrying meaningful risk, whether or not it is delivered under a specific named regulatory framework, so these documents should not be treated as optional for private commercial work (C); and producing sound, workable RoE and authorisation documentation is a genuinely shared responsibility, requiring active professional input, expertise, and accountability from the Red Team provider, not something the client can or should be left to produce entirely alone (B).