Magento comes with a new design pattern called the service contracts.
A service contract is a set of PHP interfaces used in a module. You can check an interface in any module API folder. Service contracts include service and data interfaces, which hide business logic details.
In Magento 2 You can add extension attributes for Order by creating an extension_attributes.xml file.
Extension attributes are the persistent Attribute so you can’t find extension_attributes values in a database.
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