<type name="Magento\Customer\Block\SectionConfig">
    <arguments>
        <argument name="sectionConfig" xsi:type="object">
            SectionInvalidationConfigData
        </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\Customer\Block\SectionConfig object that the following arguments should be used in the foo object's constructor.

• $sectionConfig = new SectionInvalidationConfigData (...)

<type name="Magento\Customer\Block\SectionConfig">
    <arguments>
        <argument name="clientSideSections" xsi:type="array">
            <item name="checkout-data" xsi:type="string">
                checkout-data
            </item>
        </argument>
    </arguments>
</type>

This is a type configuration.

This configuration is for the frontend area. This is the area that corresponds with the cart sub-application.

This configuration tells the Object Manager that when a user wants a Magento\Customer\Block\SectionConfig object that the following arguments should be used in the foo object's constructor.

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