<type name="Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend">
    <plugin name="attributeValidation"
            type="Magento\Catalog\Plugin\Model\Attribute\Backend\AttributeValidation"></plugin>
</type>

This is a type configuration.

This configuration is for the base area. Base area configurations will apply to all the sub-applications.

This configuration also tells the object manager to use the Magento\Catalog\Plugin\Model\Attribute\Backend\AttributeValidation plugin class with Magento\Eav\Model\Entity\Attribute\Backend\AbstractBackend objects.

<type name="Magento\Catalog\Plugin\Model\Attribute\Backend\AttributeValidation">
    <arguments>
        <argument name="allowedEntityTypes" xsi:type="array">
            <item name="product" xsi:type="string">
                Magento\Catalog\Api\Data\ProductInterface
            </item>
            <item name="category" xsi:type="string">
                Magento\Catalog\Api\Data\CategoryInterface
            </item>
        </argument>
    </arguments>
</type>

This is a type configuration.

This configuration is for the base area. Base area configurations will apply to all the sub-applications.

This configuration tells the Object Manager that when a user wants a Magento\Catalog\Plugin\Model\Attribute\Backend\AttributeValidation object that the following arguments should be used in the foo object's constructor.

• $allowedEntityTypes = [...items...];