Callouts and cross-references used to refer to particular items that are contained within the same article, such as references, tables, figures, formula, and affiliations, should be tagged using the <xref> element. Each item that is the object of a callout must have an id attribute that uniquely identifies the item within the article. The callout should be tagged using an <xref> with an attribute rid, and the value of the rid attribute should point to the id attribute of the item or items being called out. The id and rid attributes are defined respectively as ID and IDREFS, which is a mechanism of XML DTDs for linking within a document.
There are certain types of items within an article for which an id attribute is required because they should always have a callout within the article. These are:
<ref> Reference
<fn> Footnote
<fig> Figure
<table-wrap> Table
<disp-formula> Display Formula
<aff> Affiliation
<target> Link anchor or page
<milestone-start> Start of a non-hierarchically nested item and paired with <milestone-end> (infrequently used)
<underline-start> Start of a non-hierarchically nested underlining of text and paired with <underline-end> (infrequently used)
<overline-start> Start of a non-hierarchically nested overlining of text and paired with <overline-end> (infrequently used)
The JATS DTD allows an id attribute to be placed on almost any element, however, in order to provide consistent user experiences when following cross-reference links and for compatibility with older versions of JATS, we require id attributes to be placed on the elements in the above list and in general not on elements that are nested within these elements. For example, for a table the id attribute should be placed on the <table-wrap> element and not on the <oasis:table> element. In cases where it is useful to create a cross-reference link to a particular part of an item an id attribute may additionally be placed on the particular item, for example to link to a particular row within a table.
The <xref> element should contain the text of the callout, which will be rendered as a clickable link. The one exception to this is the rare situation of a cross reference that appears on an article title, in which case the <xref> must must be empty. The value of the rid attribute should be identifier(s) that correspond to the items that are being called out. The <label> element, if one is present in the item being referenced, may be used to check for consistency between the text and rid attribute of the <xref>. If a callout references more than one item, for example “Figures 1-3”, then the rid attribute should contain the id values of all relevant items in a space-separated list.
The <xref> element has an optional attribute ref-type to describe the type of item being referenced, for example a table or a figure. If the ref-type attribute is used its value must be consistent with the type of element that is referenced by the rid attribute.
Callout to a reference (year is linked):
<xref ref-type="bibr" rid="CIT0002">2016</xref>
Callout to more than one numbered reference:
<xref ref-type="bibr" rid="cit0010 cit0011 cit0012"><sup>10–12</sup></xref>
Callout to a footnote or endnote:
<xref rid="en0002" ref-type="fn"><sup>2</sup></xref>
Callout to a figure:
<xref ref-type="fig" rid="F0002">Figure 2</xref>
Callout to a table:
<xref ref-type="table" rid="t0001">Table 1</xref>
Callout to more than one table:
<xref ref-type="table" rid="t0001 t0002">Tables 1 and 2</xref>
Within a table, callout to table footnote:
<xref ref-type="table-fn" rid="t1fn0001"><sup>a</sup></xref>