For example, entry type is defined as follows:

<xs:complexType name="entryType">
<xs:sequence>
<xs:choice maxOccurs="unbounded">
<xs:annotation>
<xs:appinfo>
<jaxb:property name="items" />
</xs:appinfo>
</xs:annotation>
<xs:element ref="atom:author" />
<xs:element ref="atom:category" />
<xs:element ref="atom:content" />
<xs:element ref="atom:contributor" />
<xs:element ref="atom:id" />
<xs:element ref="atom:link" />
<xs:element ref="atom:published" />
<xs:element ref="atom:rights" />
<xs:element ref="atom:source" />
<xs:element ref="atom:summary" />
<xs:element ref="atom:title" />
<xs:element ref="atom:updated" />
</xs:choice>

<!-- Normal ATOM extension element -->
<xs:group ref="atom:extensionElement" />
</xs:sequence>
<xs:attributeGroup ref="atom:atomCommonAttributes" />
</xs:complexType>

This only enforces one of the atom elements within the choice. All other elements are accepted virtue of the <xs:group ref="atom:extensionElement" />.