Multi-tenant data isolation - critical security skill for SaaS applications. Use whenever each user or customer must see only their own data. Triggers on: row-level security, tenant isolation, "users see each other's data", "data leak", per-customer filtering, designer/owner access with data isolation. Covers three isolation patterns with tradeoffs, plus custom fields per tenant (explicit `tenant`, `tenant_fields_rights`) and sticky overrides for scheduled exports and alerts. Use eagerly - wrong isolation is a security breach, not a bug. Pair with core for token generation and data-integration for connection-level overrides.