At least in Magento 2.4.4 it's going to be
<argument name="sort_order" xsi:type="string">80</argument>
inside your custom tab block, where "80" will define the relative position of a specific tab.
So for example, your complete xml might look like this:
<?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">
<body>
<referenceBlock name="product.info.details">
<block class="Magento\Catalog\Block\Product\View" name="included_items.tab" as="included_items" template="product/view/included_tab.phtml" group="detailed_info">
<arguments>
<argument translate="true" name="title" xsi:type="string">Included Items</argument>
<argument name="sort_order" xsi:type="string">10</argument>
</arguments>
</block>
<block class="Magento\Catalog\Block\Product\View" name="sizing_chart.tab" as="sizing_chart" template="product/view/sizing_chart_tab.phtml" group="detailed_info">
<arguments>
<argument translate="true" name="title" xsi:type="string">Sizing Chart</argument>
<argument name="sort_order" xsi:type="string">50</argument>
</arguments>
</block>
</referenceBlock>
</body>
</page>
Included Items has index "10" which means it will appear to the left of "Sizing Chart" which has index "50". It's necessary to clean cache after making changes to that xml file.