Designing a multi-tenant federated GraphQL schema is crucial for supporting multiple tenants with shared and tenant-specific data. The approach to implementing multi-tenancy in a federated GraphQL API can be either transport-based or schema-based. In the transport-based approach, the tenant information is passed as part of the request, while in the schema-based approach, it's integrated into the GraphQL schema itself. Both approaches have their advantages and disadvantages, with the transport-based approach being more flexible but also error-prone and harder to govern at scale, whereas the schema-based approach is more declarative and allows for a clear contract between subgraphs, making it easier to reason about behavior and ensure harmony among them. Ultimately, the choice of approach depends on the specific requirements and constraints of the project.