I often see code like <update handle='xy' />
in layout files.
vendor\magento\module-sales\view\frontend\layout\sales_email_order_items.xml
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd" label="Email Order Items List" design_abstraction="custom">
<update handle="sales_email_order_renderers"/>
<update handle="sales_email_item_price"/>
<body>
...
</body>
</page>
What is the purpose of it?