class

<xsd:attribute name="class" use="required">
  <xsd:annotation>
    <xsd:documentation>
      <p>The class attribute defines the semantic of the arc, and influences:
        <ul>
          <li>the way that arc should be rendered,</li>
          <li>the overall syntactic validity of the map.</li>
        </ul>
      </p>
      <p>The various classes encompass all possible types of SBGN arcs:
        <ul>
          <li>production and consumption arcs,</li>
          <li>all types of modification arcs,</li>
          <li>logic arcs,</li>
          <li>equivalence arcs.</li>
        </ul>To express a reversible reaction, use production arcs on both sides of the Process Node.</p>
    </xsd:documentation>
  </xsd:annotation>
  <xsd:simpleType>
    <xsd:restriction base="xsd:string">
      <!-- PD only -->
      <xsd:enumeration value="production"/>
      <xsd:enumeration value="consumption"/>
      <xsd:enumeration value="catalysis"/>
      <!-- PD and ER -->
      <xsd:enumeration value="modulation"/>
      <xsd:enumeration value="stimulation"/>
      <xsd:enumeration value="inhibition"/>
      <!-- ER only -->
      <xsd:enumeration value="assignment"/>
      <xsd:enumeration value="interaction"/>
      <xsd:enumeration value="absolute inhibition"/>
      <xsd:enumeration value="absolute stimulation"/>
      <!-- AF only -->
      <xsd:enumeration value="positive influence"/>
      <xsd:enumeration value="negative influence"/>
      <xsd:enumeration value="unknown influence"/>
      <!-- PD and AF -->
      <xsd:enumeration value="equivalence arc"/>
      <!-- PD, ER and AF -->
      <xsd:enumeration value="necessary stimulation"/>
      <xsd:enumeration value="logic arc"/>
      <!-- NB Glyph classes are in a separate enumeration -->
    </xsd:restriction>
  </xsd:simpleType>
</xsd:attribute>