The id attribute is the QIF id of the point, used for referencing.
Source
<xs:element name="DefiningPoint" type="DefiningPointMeasurementType" maxOccurs="unbounded"><xs:annotation><xs:documentation>Each DefiningPoint element is a point on a measured point-defined curve or a measured point-defined surface.</xs:documentation></xs:annotation></xs:element>
<xs:complexType name="DefiningPointsMeasurementType"><xs:annotation><xs:documentation>The DefiningPointsMeasurementType defines a list of ordered measured feature defining points.</xs:documentation></xs:annotation><xs:sequence><xs:element name="DefiningPoint" type="DefiningPointMeasurementType" maxOccurs="unbounded"><xs:annotation><xs:documentation>Each DefiningPoint element is a point on a measured point-defined curve or a measured point-defined surface.</xs:documentation></xs:annotation></xs:element></xs:sequence><xs:attributeGroup ref="AttrPoint"><xs:annotation><xs:documentation>The AttrPoint attribute group defines accuracies common to all measured feature defining points.</xs:documentation></xs:annotation></xs:attributeGroup><xs:attributeGroup ref="AttrMeasuredPoint"><xs:annotation><xs:documentation>The AttrMeasuredPoint attribute group defines uncertainties and errors common to all measured feature defining points.</xs:documentation></xs:annotation></xs:attributeGroup><xs:attribute name="n" type="NaturalType" use="required"><xs:annotation><xs:documentation>The required n attribute is the number of measured feature defining points in the list.</xs:documentation></xs:annotation></xs:attribute></xs:complexType>
<xs:attribute name="n" type="NaturalType" use="required"><xs:annotation><xs:documentation>The required n attribute is the number of measured feature defining points in the list.</xs:documentation></xs:annotation></xs:attribute>