Extension attributes are new in Magento 2. They are used to extend functionalities and often use more complex data types than custom attributes. Extension Attributes are used to allow for customization of the strict Service Contracts. These attributes do not appear on the GUI.

Third-party developers cannot change the API Data interfaces defined in the Magento Core code. However, most of these entities have a feature called extension attributes. Check the interface for the methods getExtensionAttributes() and setExtensionAttributes() to determine if they are available for the entity.