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Why are entities such as ‘order’ in the eav_entity_type table if they are not proper EAV entities?

The eav_entity_type table contains 4 ‘proper’ EAV entities (customer, customer_address, catalog_category, catalog_product), but also some entities that are not ‘proper’ EAV entities, in the sense that they don’t have their own [x]_entity_varchar/int/text/datetime tables and their models do not implement the MagentoFrameworkApiCustomAttributesDataInterface.

Why are these entities referenced in this table?