In ServiceNow, the best practice for setting up User Criteria and Shared Knowledge Bases is to define them in the parent domain. This approach ensures that the knowledge bases are accessible to all relevant child domains, promoting efficient information sharing and management. When knowledge bases and user criteria are defined at the parent domain level, they inherit down to the child domains, allowing for centralized control while still supporting visibility across the domain hierarchy.

This practice aligns with the principles of domain separation, which is a key feature in ServiceNow for managing data and user access in a multi-tenant environment. By defining these elements in the parent domain, organizations can maintain a clear and organized structure that supports both separation and sharing of knowledge as needed.

For more detailed guidance on this topic, ServiceNow's official documentation provides insights on designing user criteria for knowledge bases, which can be found in their support portal. It is recommended to review these resources for a comprehensive understanding of the best practices in configuring user criteria and knowledge bases within ServiceNow.