XSD Schem

 <task operation="delete">
            <id>3435</id>
        </task>
        <task operation="update">
            <id>632</id>
            <state>В процессе</state>
        </task>

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
  <xs:element name="task">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="id" type="xs:string"/>
      </xs:sequence>
      <xs:attribute name="operation" type="xs:string" use="required"/>
    </xs:complexType>
  </xs:element>
</xs:schema>

<xs:element maxOccurs="unbounded" name="task">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="id" type="xs:unsignedInt" />
      <xs:element minOccurs="0" name="state" type="xs:string" />
    </xs:sequence>
    <xs:attribute name="operation" type="xs:string" use="required" />
  </xs:complexType>
</xs:element>

<xs:element maxOccurs="unbounded" name="task">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="id" type="xs:unsignedInt" />
      <xs:element minOccurs="0" name="state" type="xs:string" />
    </xs:sequence>
    <xs:attribute name="operation" type="taskOperation" use="required" />
  </xs:complexType>
</xs:element>
 
<xs:simpleType name="taskOperation">
  <xs:restriction base="xs:string">
    <xs:enumeration value="delete" />
    <xs:enumeration value="update" />
  </xs:restriction>
</xs:simpleType>