Instead of just
including a block
into an email template, Magento makes it possible to load a hole tree of blocks or just in other words a layout.
Here a small post about the the layout directive in Magento 2 email templates with source code examples
from Magento.
A pretty popular example can be found inside of the Magento_Sales::email/invoice_new.html template.
{{layout area="frontend" handle="sales_email_order_invoice_items" invoice=$invoice order=$order}}
The keyword area is needed to find the right XML files for the handle. But in the case
from above it is superfluous, cause Magento will always set area frontend if no area is given.
The keyword handle will guide Magento to all XML files that have a path like
<vendor_name>/<module_name>/view/frontend/layout/sales_email_order_invoice_items.xml
for example for the example from above. All those XML files will be merged the same way as normal layouts XML files.
Here for example the content of sales_email_order_invoice_items.xml contained in the Magento Sales
module.
<?xml version="1.0"?>
<!--
/**
* Copyright © 2016 Magento. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd" label="Email Invoice Items List" design_abstraction="custom">
<update handle="sales_email_order_invoice_renderers"/>
<update handle="sales_email_item_price"/>
<body>
<block class="Magento\Sales\Block\Order\Email\Invoice\Items" name="items" template="email/invoice/items.phtml">
<block class="Magento\Framework\View\Element\RendererList" name="sales.email.order.invoice.renderers" as="renderer.list"/>
<block class="Magento\Sales\Block\Order\Invoice\Totals" name="invoice_totals" template="order/totals.phtml" cacheable="false">
<arguments>
<argument name="label_properties" xsi:type="string">colspan="2"</argument>
</arguments>
<block class="Magento\Tax\Block\Sales\Order\Tax" name="tax" template="order/tax.phtml"/>
</block>
</block>
<block class="Magento\Framework\View\Element\Template" name="additional.product.info" template="Magento_Theme::template.phtml"/>
</body>
</page>
We have order and invoice variables in the example above. Those variables will be
accessible for the root block of the layout with magic getters like the following example from
Magento\Sales\Block\Order\Email\Invoice\Items::_prepareItem.
/**
* Prepare item before output
*
* @param \Magento\Framework\View\Element\AbstractBlock $renderer
* @return \Magento\Sales\Block\Items\AbstractItems
*/
protected function _prepareItem(\Magento\Framework\View\Element\AbstractBlock $renderer)
{
$renderer->getItem()->setOrder($this->getOrder());
$renderer->getItem()->setSource($this->getInvoice());
}