Disconnected systems usually fail at the handoff, not at the data model.
A team can have the right fields, the right endpoint, and still end up with brittle work if each system is unsure who is calling, what it is allowed to do, or how to recover when the request is interrupted. That is why API integration is as much an authorization problem as a plumbing problem.
OAuth 2.0 is the common pattern for delegated access: one application acts on behalf of another without sharing a password. In practice, that means a workflow can be given a narrow permission set instead of broad, standing access. For service-to-service calls, some platforms also expect explicit security objects or connected-app setup so the gateway knows which caller is trusted and what data it may touch.
The design choice is usually this: do you want a one-off connection that works today, or an integration you can support six months from now? The second option requires clearer boundaries. Authentication, registration, and documentation need to be visible enough that an operations team can review them, troubleshoot them, and change them without guessing.
A useful rule: if the integration cannot explain its own access path in plain English, it will probably be expensive to maintain.
That is also why interoperability rules in regulated settings increasingly ask for technical API documentation, capability statements, and authorization details up front. When the access model is clear, adoption is easier and the risk of surprise is lower. When it is not, every exception becomes a support issue.
The best integrations do not just move data. They make trust explicit.