Context: How does \Magento\Framework\DataObject\Copy::copyFieldsetToTarget()
work?
app/code/Magento/Quote/etc/fieldset.xml#L343-L389
<fieldset id="order_address">
<field name="prefix">
<aspect name="to_customer_address" />
</field>
<field name="firstname">
<aspect name="to_customer_address" />
</field>
<field name="middlename">
<aspect name="to_customer_address" />
</field>
<field name="lastname">
<aspect name="to_customer_address" />
</field>
<field name="suffix">
<aspect name="to_customer_address" />
</field>
<field name="company">
<aspect name="to_customer_address" />
</field>
<field name="street">
<aspect name="to_customer_address" />
</field>
<field name="city">
<aspect name="to_customer_address" />
</field>
<field name="region">
<aspect name="to_customer_address" />
</field>
<field name="region_id">
<aspect name="to_customer_address" />
</field>
<field name="postcode">
<aspect name="to_customer_address" />
</field>
<field name="country_id">
<aspect name="to_customer_address" />
</field>
<field name="telephone">
<aspect name="to_customer_address" />
</field>
<field name="fax">
<aspect name="to_customer_address" />
</field>
<field name="email">
<aspect name="to_customer_address" />
</field>
</fieldset>
app/code/Magento/Sales/etc/fieldset.xml#L772-L833
<fieldset id="order_address">
<field name="prefix">
<aspect name="to_customer" />
<aspect name="to_customer_address" />
</field>
<field name="firstname">
<aspect name="to_customer" />
<aspect name="to_customer_address" />
</field>
<field name="middlename">
<aspect name="to_customer" />
<aspect name="to_customer_address" />
</field>
<field name="lastname">
<aspect name="to_customer" />
<aspect name="to_customer_address" />
</field>
<field name="suffix">
<aspect name="to_customer" />
<aspect name="to_customer_address" />
</field>
<field name="email">
<aspect name="to_customer" />
<aspect name="to_customer_address" />
</field>
<field name="customer_dob">
<aspect name="to_customer" targetField="dob" />
</field>
<field name="customer_taxvat">
<aspect name="to_customer" targetField="taxvat" />
</field>
<field name="customer_gender">
<aspect name="to_customer" targetField="gender" />
</field>
<field name="company">
<aspect name="to_customer_address" />
</field>
<field name="street">
<aspect name="to_customer_address" />
</field>
<field name="city">
<aspect name="to_customer_address" />
</field>
<field name="region">
<aspect name="to_customer_address" />
</field>
<field name="region_id">
<aspect name="to_customer_address" />
</field>
<field name="postcode">
<aspect name="to_customer_address" />
</field>
<field name="country_id">
<aspect name="to_customer_address" />
</field>
<field name="telephone">
<aspect name="to_customer_address" />
</field>
<field name="fax">
<aspect name="to_customer_address" />
</field>
</fieldset>