Engineering change orders are a critical aspect of design and process improvements. The description of the ECO has been echoed by many of the previous posts. ECOs are often critical controlled documents that enable changes to designs or processes to be appropriately documented. In my personal experience, ECOs are often initiated as the result of non-conformence events (NCEs) and corrective actions and preventative actions (CAPAs). While not all NCEs and CAPAs require an ECO, many of them do in order to officially document critical changes. Many NCEs often lead to CAPAs, and all CAPA documents in my company usually designate a space for an ECO reference so that continuity can be maintained and documented.