rail:annotationRef 

refers to the id attribute of an annotation element

http://wiki.railml.org/index.php?title=TT:annotationRef_stopDescription

Element Information

Model

Attributes

QName Type Fixed Default Use Inheritable Annotation
operatingPeriodRef rail:tGenericRef optional 
references a reduction of the operating days of the annotation, must be a subset of the parents operating days
ref rail:tGenericRef required 
reference is required because it's the purpose of the element
Wildcard: ANY attribute from ANY namespace OTHER than 'http://www.railml.org/schemas/2016'

Source 

<xs:element minOccurs="0" maxOccurs="unbounded" name="annotationRef" type="rail:tAnnotationRef">
  <xs:annotation>
    <xs:documentation>refers to the id attribute of an annotation element</xs:documentation>
    <xs:documentation source="http://wiki.railml.org/index.php?title=TT:annotationRef_stopDescription"/>
  </xs:annotation>
</xs:element>

Sample