NCBI element xref

Abstract	false (This can be used in an instance)
Nillable	false (Instance values cannot use xsi:nil)
Declared Namespaces	xmlns:xsi=http://www.w3.org/2001/XMLSchema-instance xmlns:xlink=http://www.w3.org/1999/xlink xmlns:xs=http://www.w3.org/2001/XMLSchema xmlns:mml=http://www.w3.org/1998/Math/MathML

<xs:element name="xref">
    <xs:complexType mixed="true">
        <xs:choice minOccurs="0" maxOccurs="unbounded">
            <xs:element ref="bold"/>
            <xs:element ref="italic"/>
            <xs:element ref="monospace"/>
            <xs:element ref="named-content"/>
            <xs:element ref="overline"/>
            <xs:element ref="overline-start"/>
            <xs:element ref="overline-end"/>
            <xs:element ref="sc"/>
            <xs:element ref="strike"/>
            <xs:element ref="underline"/>
            <xs:element ref="underline-start"/>
            <xs:element ref="underline-end"/>
            <xs:element ref="inline-graphic"/>
            <xs:element ref="private-char"/>
            <xs:element ref="graphic"/>
            <xs:element ref="media"/>
            <xs:element ref="sub"/>
            <xs:element ref="sup"/>
        </xs:choice>
        <xs:attribute name="id" type="xs:ID"/>
        <xs:attribute name="ref-type">
            <xs:simpleType>
                <xs:restriction base="xs:token">
                    <xs:enumeration value="aff"/>
                    <xs:enumeration value="app"/>
                    <xs:enumeration value="author-notes"/>
                    <xs:enumeration value="bibr"/>
                    <xs:enumeration value="book-part"/>
                    <xs:enumeration value="boxed-text"/>
                    <xs:enumeration value="chem"/>
                    <xs:enumeration value="contrib"/>
                    <xs:enumeration value="corresp"/>
                    <xs:enumeration value="disp-formula"/>
                    <xs:enumeration value="fig"/>
                    <xs:enumeration value="fn"/>
                    <xs:enumeration value="glossary"/>
                    <xs:enumeration value="kwd"/>
                    <xs:enumeration value="list"/>
                    <xs:enumeration value="plate"/>
                    <xs:enumeration value="scheme"/>
                    <xs:enumeration value="sec"/>
                    <xs:enumeration value="statement"/>
                    <xs:enumeration value="supplementary-material"/>
                    <xs:enumeration value="table"/>
                    <xs:enumeration value="table-fn"/>
                    <xs:enumeration value="other"/>
                </xs:restriction>
            </xs:simpleType>
        </xs:attribute>
        <xs:attribute name="rid" type="xs:IDREFS"/>
    </xs:complexType>
</xs:element>

element {}xref

Source