Key clustering happens when a plaintext message generates identical ciphertext messages using the same transformation algorithm, but with different keys. Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 4: Cryptography (page 130).

The document that explains the proper use of the organization's assets is the acceptable use policy. An acceptable use policy is a document that defines the rules and guidelines for the appropriate and responsible use of the organization's information systems and resources, such as computers, networks, or devices. An acceptable use policy can help to prevent or reduce the misuse, abuse, or damage of the organization's assets, and to protect the security, privacy, and reputation of the organization and its users. An acceptable use policy can also specify the consequences or penalties for violating the policy, such as disciplinary actions, termination, or legal actions. A human resources policy, a code of ethics, and an access control policy are not the documents that explain the proper use of the organization's assets, as they are related to the management, values, or authorization of the organization's employees or users, not the usage or responsibility of the organization's information systems or resources. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 1, Security and Risk Management, page 47. Official (ISC)2 CISSP CBK Reference, Fifth Edition, Chapter 1, Security and Risk Management, page 62.