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Integration
A connection to another system that has its own rules and its own opinions.
Your billing system, your CRM, your identity provider, a partner's API. Each has its own idea of what a customer is, its own failure modes, and its own idea of what it will let you do. Most of the real work in an automation project is here, not in the model.
Why it matters
Every integration is a place where two systems can disagree about the truth.