You can give TAB'S priority(position) like below:
<referenceBlock name="product.info.details">
<referenceBlock name="product.info.description">
<arguments>
<argument name="priority" xsi:type="string">1</argument>. <!-- you can pass position here -->
</arguments>
</referenceBlock>
<referenceBlock name="product.attributes">
<arguments>
<argument name="priority" xsi:type="string">3</argument>
</arguments>
</referenceBlock>
<referenceBlock name="reviews.tab">
<arguments>
<argument name="priority" xsi:type="string">4</argument>
</arguments>
</referenceBlock>
<!-- MY OWN CUSTOM BLOCK ON THE SECOND POSITION -->
<block class="Magento\Catalog\Block\Product\View\Description" name="product.features" as="features" template="product/view/features.phtml" group="detailed_info">
<arguments>
<argument translate="true" name="title" xsi:type="string">Features</argument>
<argument name="priority" xsi:type="string">2</argument>
</arguments>
</block>
<!-- MY OWN CUSTOM BLOCK ENDS HERE -->
</referenceBlock>