Custom field sections are an optional feature that allow you to group your custom fields into sections with a bold heading. It can make them easier to read for your workers.
Instead of sending an array of custom fields, you send an array of sections. Each section then has an array of custom fields inside it, like so:
<sections type="array">
<section>
<title>Section One - Customer Details</title>
<custom_fields type="array">
<custom_field>
<name>Customer Name</name>
<value>J SMith</value>
</custom_field>
</custom_fields>
</section>
<section>
<title>Section Two - Job Details</title>
<custom_fields type="array">
<custom_field>
<name>Job Type</name>
<value>Premium Housewash</value>
</custom_field>
</custom_fields>
</section>
</sections>