Multi-tenant authorization is a model for managing user permissions across multiple accounts, organizations, or groups, offering isolated environments for each tenant with tailored access controls. By combining it with Role-Based Access Control (RBAC), which assigns users predefined roles dictating their permissions, the limitations of RBAC, such as role explosion and lack of granularity, can be addressed. The text highlights the importance of multi-tenant authorization for applications where users belong to multiple independent environments, emphasizing its role in ensuring that permissions are properly scoped within isolated environments. It also discusses the benefits of extending RBAC with Relationship-Based Access Control (ReBAC) for more nuanced permissions based on user-resource relationships. Permit.io is introduced as a tool to efficiently implement multi-tenant authorization, offering centralized policy management, dynamic role assignments, and support for ReBAC to create a scalable and secure user permission framework across different tenants.