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<xref> X (cross) Reference

Reference to an object within the document (for example, a table, a bibliographic citation).

Usage/Remarks

This element may be used to reference anything that has an attribute of type @id. This single element takes the place of the many named types of references (such as figure references, table references, and footnote references) that are common in many tag sets.

Best Practice

The @ref-type attribute (of the <xref> element) can be used to preserve information concerning what type of element is being pointed to by the cross-reference.
Accessibility
Sometimes a <xref> needs to be pronounced in a way that is not reflected in its content or its tagging. The @alt attribute can be used to record the pronunciation for screen readers and other accessibility devices. For example, the element cross-reference to a figure that follows might be pronounced as “Figure 4”: 
... See <xref rid="f4" alt="figure 4">Fig&#x00A0;IV.</xref> ...
Display/Formatting Note
The content of the reference (if present) will be displayed as the link.
Attributes
alt
custom-type
ref-type
rid
specific-use

Base Attributes

id
xml:base
xml:lang
Models and Context
May be contained in
<abbrev>, <addr-line>, <aff>, <alt-title>, <anonymous>, <article-title>, <attrib>, <award-id>, <bold>, <book-title>, <book-volume-id>, <book-volume-number>, <chapter-title>, <chem-struct>, <code>, <collab>, <comment>, <compound-kwd-part>, <conf-acronym>, <conf-loc>, <conf-name>, <conf-num>, <conf-sponsor>, <conf-theme>, <contrib>, <contrib-group>, <copyright-statement>, <corresp>, <def-head>, <degrees>, <disp-formula>, <edition>, <element-citation>, <email>, <etal>, <ext-link>, <fax>, <fig>, <fig-group>, <fixed-case>, <funding-source>, <funding-statement>, <given-names>, <gov>, <graphic>, <inline-formula>, <inline-supplementary-material>, <institution>, <issue>, <issue-part>, <issue-title>, <italic>, <kwd>, <label>, <license-p>, <media>, <meta-name>, <meta-value>, <mixed-citation>, <monospace>, <name-address-wrap>, <named-content>, <nav-pointer>, <on-behalf-of>, <overline>, <p>, <part-title>, <patent>, <phone>, <prefix>, <preformat>, <product>, <publisher-loc>, <publisher-name>, <rb>, <related-article>, <related-object>, <role>, <roman>, <sans-serif>, <sc>, <see>, <see-also>, <see-also-entry>, <see-entry>, <self-uri>, <series>, <serif>, <sig>, <sig-block>, <source>, <speaker>, <std-organization>, <strike>, <string-conf>, <string-date>, <string-name>, <styled-content>, <sub>, <subject>, <subtitle>, <suffix>, <sup>, <supplement>, <supplementary-material>, <support-source>, <surname>, <table-wrap>, <table-wrap-group>, <target>, <td>, <term>, <term-head>, <th>, <title>, <trans-source>, <trans-subtitle>, <trans-title>, <underline>, <unstructured-kwd-group>, <uri>, <verse-line>, <version>, <volume>, <volume-id>, <volume-series>, <volume-title>, <xref>
Description
Any combination of:
Content Model
<!ELEMENT  xref         (#PCDATA %xref-elements;)*                   >
Expanded Content Model

(#PCDATA | email | ext-link | uri | inline-supplementary-material | related-article | related-object | hr | bold | fixed-case | italic | monospace | overline | overline-start | overline-end | roman | sans-serif | sc | serif | strike | underline | underline-start | underline-end | ruby | alternatives | inline-graphic | inline-media | private-char | chem-struct | inline-formula | tex-math | mml:math | abbrev | index-term | index-term-range-end | milestone-end | milestone-start | named-content | styled-content | fn | target | xref | sub | sup | x | break)*

Tagged Samples
Link to bibliographic references 
...
<book-part>
 <book-part-meta>...</book-part-meta>
 <body>
  <p>... Although there is considerable descriptive literature on day hospital
   care,<xref ref-type="bibr" rid="B1">1</xref> concern has been expressed
   that evidence for effectiveness is equivocal and that day hospital care
   is expensive.<xref ref-type="bibr" rid="B2">2</xref> ...</p>
  ...
 </body>
 <back>
  ...
  <ref-list>
   <ref id="B1"><label>1</label>
    <element-citation publication-type="book">
     <collab>Research Unit of the Royal College of Physicians 
      and British Geriatric Society</collab>
     <source>Geriatric day hospitals: their role and guidelines 
      for good practice</source>
     <publisher-loc>London</publisher-loc>
     <publisher-name>RCP</publisher-name>
     <year iso-8601-date="1994">1994</year>
    </element-citation>
   </ref>
   <ref id="B2">...</ref>
   ...
  </ref-list>
  ...
 </back>
</book-part>
...
Link to figures 
...
<sec id="bid.36">
 <title>Microbial Genomes</title>
 <p>... A CON entry, containing instructions on how to put
  the pieces back together, is also made. The CON entry
  contains descriptor information, such as source organism
  and references, as well as a join statement providing
  explicit instructions on how to generate the complete
  genome from the pieces. The Accession number assigned 
  to the CON record is also added as a secondary Accession
  number on each of the pieces that make up the complete
  genome (see <xref ref-type="fig" rid="bid.37">Figure 2</xref>).
  <fig id="bid.37">
   <label>2</label>
   <caption>
    <title>A GenBank CON entry for a complete bacterial 
     genome.</title>
    <p>The information toward the <italic>bottom</italic> of the 
     record describes how to generate the complete genome
     from the pieces.</p>
   </caption>
   <graphic xmlns:xlink="http://www.w3.org/1999/xlink"
     xlink:href="ch1f2" mimetype="image/jpeg"/>
  </fig>
 </p>
 ...
</sec>
...
Cross reference with @alt in figures 
...
<sec id="sec3">
 ...
 <p>Allocation of tags to individual molecules is outlined in
  <xref ref-type="fig" rid="F2" alt="figure two">Fig.&#x00A0;2</xref>.
  First, the complete repertoire of tags in a plasmid library is 
  ligated to the entire population of cDNAs ... After separating 
  loaded microbeads from unloaded microbeads by FACS, as shown in 
  <xref ref-type="fig" rid="F4" alt="figure four">Fig.&#x00A0;4</xref>, 
  the hybridized DNA is ligated to the anti-tag, covalently attaching 
  one strand of the DNA to the microbead's surface. This permits easy 
  removal of the noncovalently attached strand.</p>
 <fig id="F2">...</fig>
 <fig id="F3">...</fig>
 <fig id="F4">...</fig>
</sec>
...
Linking authors to both affiliations and author notes 
...
<book-part-meta>
 ...
 <contrib-group>
  <contrib contrib-type="author">  
   <name-alternatives>
    <name name-style="western"><surname>Zhang</surname>
     <given-names>Y. P.</given-names></name>
    <string-name name-style="eastern" xml:lang="zh">张轶泼</string-name>
   </name-alternatives>
   <xref ref-type="aff" rid="a1">1</xref>
   <xref ref-type="author-notes" rid="n1">a)</xref>
  </contrib>
  <contrib contrib-type="author">
   <name-alternatives>
    <name name-style="western"><surname>Isobe</surname>
     <given-names>M.</given-names></name>
    <string-name name-style="eastern" xml:lang="zh">磯部光孝</string-name>
   </name-alternatives>
   <xref ref-type="aff" rid="a2">2</xref>
  </contrib>
  <contrib contrib-type="author">
   <name-alternatives>
    <name name-style="western"><surname>Liu</surname>
     <given-names>Yi</given-names></name>
    <string-name name-style="eastern" xml:lang="zh">刘仪</string-name>
   </name-alternatives>
   <xref ref-type="aff" rid="a1">1</xref>
  </contrib>
 </contrib-group>
 <aff id="a1">...</aff>
 <aff id="a2">...</aff>
 <author-notes id="n1">...</author-notes>
</book-part-meta>
...
Link to tables 
...
<table-wrap id="c5-tab2">...</table-wrap>
<table-wrap id="c5-tab3">...</table-wrap>
<p>... (see <xref ref-type="table" rid="c5-tab2">Tables&#x00A0;2</xref> 
and <xref ref-type="table" rid="c5-tab3">3</xref>)</p>
...
Link to section 
...
<p>... see <xref ref-type="sec" rid="c5-s3">Section&#x00A0;3</xref></p>
<sec id="c5-s3">
 <title>...</title>
 ...</sec>
...
Link to footnote 
...
<xref ref-type="fn" rid="int-fn1">1</xref>
...
<p>...<fn id="int-fn1"><p>...</p></fn>...</p>
...
Link to table footnote 
...
<table-wrap>
 <table>...
  <tr><td><xref ref-type="table-fn" rid="tab1-fn1">a</xref>
   </td></tr>
  ...</table>
 <table-wrap-foot>
  <fn id="tab1-fn1"><p>...</p></fn>
 </table-wrap-foot>
</table-wrap>
...
Table linked to data from which the table is derived 
...
<table-wrap id="table1">
 <label>Table 1</label>
 <caption>
  <p>Steady-state kinetic analyses for each IGF1R protein.</p>
 </caption>
 ...
 <xref ref-type="supplementary-material" rid="source-data-3" />
</table-wrap>
...
<supplementary-material id="source-data-3" content-type="sdata" 
  mimetype="text" mime-subtype="xlsx" xlink:href="source-data-3.xlsx">
 <label>Supplementary File 3</label>
 <caption>
  <title>Representative curves of steady-state kinetic analyses 
   for each IGF1R protein characterized</title>
  <p>Each data point was performed in duplicate and is shown 
   separately.</p>
 </caption>
</supplementary-material>
...