Complex sentence: An Engineering Change Notice, or Change Notice, is "a document approved by the design activity that describes and authorizes the implementation of an engineering change to the product and its approved configuration documentation".
Split into simpler sentences: An Engineering Change Notice (ECN), or Change Notice, is a document which records or authorises a change to a design. "(A) document approved by the design activity that describes and authorizes the implementation of an engineering change to the product and its approved configuration documentation".