When i use the choice/sequence with maxOccurs inside a xs:group element and a 
i try to validate an instance document(shown below with schema),
the following error is reported by xerces:
"s4s-att-not-allowed:Attribute 'minOccurs' cannot appear in element 'sequence'.

The schema-schema seems to allow this, to the best of my understanding.

Here is the schema:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
elementFormDefault="qualified" attributeFormDefault="unqualified">
	<xs:group name="custGroup">
		<xs:sequence minOccurs="0">
			<xs:element name="customer" type="xs:string"/>
			<xs:element name="orderdetails" type="xs:string"/>
			<xs:element name="billto" type="xs:string"/>
			<xs:element name="shipto" type="xs:string"/>
		</xs:sequence>
	</xs:group>
	<xs:element name="order" type="ordertype"/>
	<xs:complexType name="ordertype">
		<xs:group ref="custGroup"/>
		<xs:attribute name="status" type="xs:string"/>
	</xs:complexType>
</xs:schema>

And here is the instance document: 
<?xml version="1.0" encoding="UTF-8"?>
<order xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:noNamespaceSchemaLocation="test1.xsd" status="String">
	<customer>String</customer>
	<orderdetails>String</orderdetails>
	<billto>String</billto>
	<shipto>String</shipto>
</order>