This is a free form text field to save notes on an order.
Source
<xs:elementminOccurs="0"name="BillingComments"nillable="true"type="xs:string"xmlns:xs="http://www.w3.org/2001/XMLSchema"><xs:annotation><xs:appinfo/><xs:documentation><summary>This is a free form text field to save notes on an order.</summary></xs:documentation></xs:annotation></xs:element>