<type name="Magento\Sales\Model\Order\Pdf\Config">
    <arguments>
        <argument name="dataStorage" xsi:type="object">
            pdfConfigDataStorage
        </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\Sales\Model\Order\Pdf\Config object that the following arguments should be used in the foo object's constructor.

• $dataStorage = new pdfConfigDataStorage (...)

<virtualType name="pdfConfigDataStorage"
             type="Magento\Framework\Config\Data">
    <arguments>
        <argument name="reader" xsi:type="object">
            Magento\Sales\Model\Order\Pdf\Config\Reader
        </argument>
        <argument name="cacheId" xsi:type="string">
            sales_pdf_config
        </argument>
    </arguments>
</virtualType>

This is a virtual type configuration.

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

This virtualType configuration tells the Object Manager when a user wants a virtual pdfConfigDataStorage object that it should return a Magento\Framework\Config\Data object.

This configuration tells the Object Manager that when a user wants a pdfConfigDataStorage object that the following arguments should be used in the foo object's constructor.

• $reader = new Magento\Sales\Model\Order\Pdf\Config\Reader (...)
• $cacheId = "sales_pdf_config";