nlm-citation 

<div xmlns="http://www.w3.org/1999/xhtml">
  <h3>Nlm Citation Model</h3>
</div>

Element Information

Model

(person-group | collab) , (article-title | trans-title) , source{0,1} , patent{0,1} , trans-source{0,1} , year{0,1} , ((month{0,1} , day{0,1} , time-stamp{0,1}) | season{0,1}) , access-date{0,1} , volume{0,1} , edition{0,1} , conf-name{0,1} , conf-date{0,1} , conf-loc{0,1} , (issue | supplement) , publisher-loc{0,1} , publisher-name{0,1} , fpage{0,1} , lpage{0,1} , page-count{0,1} , series{0,1} , comment* , pub-id* , annotation{0,1}
Children: access-date, annotation, article-title, collab, comment, conf-date, conf-loc, conf-name, day, edition, fpage, issue, lpage, month, page-count, patent, person-group, pub-id, publisher-loc, publisher-name, season, series, source, supplement, time-stamp, trans-source, trans-title, volume, year

Attributes

QName Type Fixed Default Use Inheritable Annotation
id xsd:ID optional
publication-format xsd:string optional
publication-type xsd:string optional
publisher-type xsd:string optional
specific-use xsd:string optional
xlink:actuate restriction of string optional
xlink:href anyURI optional
xlink:role string optional
xlink:show restriction of string optional
xlink:title string optional
xlink:type restriction of string simple optional
xml:base xs:anyURI optional 
<div>
  <h3>base (as an attribute name)</h3>
  <p>denotes an attribute whose value provides a URI to be used as the base for interpreting any relative URIs in the scope of the element on which it appears; its value is inherited. This name is reserved by virtue of its definition in the XML Base specification.</p>
  <p>See
    <a href="http://www.w3.org/TR/xmlbase/">http://www.w3.org/TR/xmlbase/</a>for information about this attribute.</p>
</div>
xml:lang union of(xs:language, restriction of xs:string) optional 
<div>
  <h3>lang (as an attribute name)</h3>
  <p>denotes an attribute whose value is a language code for the natural language of the content of any element; its value is inherited. This name is reserved by virtue of its definition in the XML specification.</p>
</div>
<div>
  <h4>Notes</h4>
  <p>Attempting to install the relevant ISO 2- and 3-letter codes as the enumerated possible values is probably never going to be a realistic possibility.</p>
  <p>See BCP 47 at
    <a href="http://www.rfc-editor.org/rfc/bcp/bcp47.txt">http://www.rfc-editor.org/rfc/bcp/bcp47.txt</a>and the IANA language subtag registry at
    <a href="http://www.iana.org/assignments/language-subtag-registry">http://www.iana.org/assignments/language-subtag-registry</a>for further information.</p>
  <p>The union allows for the 'un-declaration' of xml:lang with the empty string.</p>
</div>

Source 

<xsd:element name="nlm-citation">
  <xsd:annotation>
    <xsd:documentation>
      <div xmlns="http://www.w3.org/1999/xhtml">
        <h3>Nlm Citation Model</h3>
      </div>
    </xsd:documentation>
  </xsd:annotation>
  <xsd:complexType>
    <xsd:sequence>
      <xsd:choice minOccurs="0" maxOccurs="unbounded">
        <xsd:element ref="person-group"/>
        <xsd:element ref="collab"/>
      </xsd:choice>
      <xsd:choice minOccurs="0" maxOccurs="unbounded">
        <xsd:element ref="article-title"/>
        <xsd:element ref="trans-title"/>
      </xsd:choice>
      <xsd:element ref="source" minOccurs="0"/>
      <xsd:element ref="patent" minOccurs="0"/>
      <xsd:element ref="trans-source" minOccurs="0"/>
      <xsd:element ref="year" minOccurs="0"/>
      <xsd:choice>
        <xsd:sequence>
          <xsd:element ref="month" minOccurs="0"/>
          <xsd:element ref="day" minOccurs="0"/>
          <xsd:element ref="time-stamp" minOccurs="0"/>
        </xsd:sequence>
        <xsd:element ref="season" minOccurs="0"/>
      </xsd:choice>
      <xsd:element ref="access-date" minOccurs="0"/>
      <xsd:element ref="volume" minOccurs="0"/>
      <xsd:element ref="edition" minOccurs="0"/>
      <xsd:element ref="conf-name" minOccurs="0"/>
      <xsd:element ref="conf-date" minOccurs="0"/>
      <xsd:element ref="conf-loc" minOccurs="0"/>
      <xsd:choice minOccurs="0" maxOccurs="unbounded">
        <xsd:element ref="issue"/>
        <xsd:element ref="supplement"/>
      </xsd:choice>
      <xsd:element ref="publisher-loc" minOccurs="0"/>
      <xsd:element ref="publisher-name" minOccurs="0"/>
      <xsd:sequence minOccurs="0" maxOccurs="unbounded">
        <xsd:element ref="fpage" minOccurs="0"/>
        <xsd:element ref="lpage" minOccurs="0"/>
      </xsd:sequence>
      <xsd:element ref="page-count" minOccurs="0"/>
      <xsd:element ref="series" minOccurs="0"/>
      <xsd:element ref="comment" minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element ref="pub-id" minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element ref="annotation" minOccurs="0"/>
    </xsd:sequence>
    <xsd:attribute name="id" use="optional" type="xsd:ID"/>
    <xsd:attribute name="publication-format" use="optional" type="xsd:string"/>
    <xsd:attribute name="publication-type" use="optional" type="xsd:string"/>
    <xsd:attribute name="publisher-type" use="optional" type="xsd:string"/>
    <xsd:attribute name="specific-use" use="optional" type="xsd:string"/>
    <xsd:attribute ref="xlink:actuate" use="optional"/>
    <xsd:attribute ref="xlink:href" use="optional"/>
    <xsd:attribute ref="xlink:role" use="optional"/>
    <xsd:attribute ref="xlink:show" use="optional"/>
    <xsd:attribute ref="xlink:title" use="optional"/>
    <xsd:attribute ref="xlink:type" use="optional" fixed="simple"/>
    <xsd:attribute ref="xml:base" use="optional"/>
    <xsd:attribute ref="xml:lang" use="optional"/>
  </xsd:complexType>
</xsd:element>

Sample