When to use Extension Attributes Instead of Custom Attributes?
Use extension attributes in Magento 2 when you need to send extra data through APIs or service contracts. They help add data to orders without changing core code. Use custom attributes when you only need to store extra data in the database. If you’re using the Magento 2 Order Attributes Extension, choose extension attributes for API use. For simple store data, custom attributes work best. This keeps your Magento 2 store fast and clean.