This is invoiceschema.xsd which imports po.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns="http://www.skatestown.com/ns/invoice"
    targetNamespace="http://www.skatestown.com/ns/invoice"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    xmlns:po="http://www.skatestown.com/ns/po">

   <xsd:import namespace="http://www.skatestown.com/ns/po"
      schemaLocation="d:\temp\po.xsd"/>

   <xsd:annotation>
      <xsd:documentation xml:lang="en">
         Invoice schema for SkatesTown.
      </xsd:documentation>
   </xsd:annotation>

   <xsd:element name="invoice" type="invoiceType"/>

   <xsd:complexType name="invoiceType">
      <xsd:sequence>
         <xsd:element name="billTo" type="po:addressType"/>
         <xsd:element name="shipTo" type="po:addressType"/>
         <xsd:element name="order">
            <xsd:complexType>
               <xsd:sequence>
                  <xsd:element name="item" type="itemType"
                               maxOccurs="unbounded"/>
               </xsd:sequence>
            </xsd:complexType>
         </xsd:element>
         <xsd:element name="tax" type="priceType"/>
         <xsd:element name="shippingAndHandling" type="priceType"/>
         <xsd:element name="totalCost" type="priceType"/>
      </xsd:sequence>
      <xsd:attribute name="id" use="required"
                     type="xsd:positiveInteger"/>
      <xsd:attribute name="submitted" use="required"
                     type="xsd:date"/>
      <xsd:attribute name="customerId" use="required"
                     type="xsd:positiveInteger"/>
   </xsd:complexType>

   <xsd:complexType name="itemType">
      <xsd:complexContent>
         <xsd:extension base="po:itemType">
            <xsd:attribute name="unitPrice" use="required"
                           type="priceType"/>
         </xsd:extension>
      </xsd:complexContent>
   </xsd:complexType>

   <xsd:simpleType name="priceType">
      <xsd:restriction base="xsd:decimal">
         <xsd:minInclusive value="0"/>
      </xsd:restriction>
   </xsd:simpleType>

</xsd:schema>