An alteration made to an AVL or BOM, such as the replacement of one component by a substitute component. Any changes should be prepared, approved and incorporated promptly and correctly to minimise problems. Changes often vary in complexity and urgency, but can have a ripple effect. Any ECO should be documented on a blank form or template that contains key information, such as a description of change, a reason for the change, the type of change and the implementation date (e.g., immediate action or implement when possible).