id 

The value of attribute id in category citation must uniquely identify a record in the
 CITATION list.

The attribute id in category citation 'primary' should be used to indicate the
 citation that the author(s) consider to be the most pertinent to
the contents of the data block.

Note that this item need not be a number; it can be any unique
identifier.
primary
1
2

Attribute Information

Used By

Element PDBx:citationType/PDBx:citation

Source 

<xsd:attribute name="id" use="required" type="xsd:string">
  <xsd:annotation>
    <xsd:documentation xml:lang="en">The value of attribute id in category citation must uniquely identify a record in the CITATION list. The attribute id in category citation 'primary' should be used to indicate the citation that the author(s) consider to be the most pertinent to the contents of the data block. Note that this item need not be a number; it can be any unique identifier. primary 1 2</xsd:documentation>
  </xsd:annotation>
</xsd:attribute>

Sample