Indirect key-based content reuse
The @conkeyref attribute provides an indirect content reference to
    topic elements, map elements, or elements within maps or topics. It allows the referencing
    element to use a key in place of a URI reference when referencing
    another topic or map.
When the target of a content reference is a topic or map, the value of the
        @conkeyref attribute is a key name that directly resolves to that topic or
      map.
When the target of a content reference is an element within a topic or map, the value of
        the @conkeyref attribute is a key name, followed by a slash ("/"), followed
        by the ID of the referenced element. The key name must be bound to the topic or map that
        contains the referenced element.
When the key name specified by the @conkeyref attribute is not defined and
        the element also specifies a @conref attribute, the @conref
        attribute is used to determine the referenced element. If no @conref
        attribute is specified there is no content reference relationship. 
The @conrefend attribute, which defines the end of a conref range, cannot
        include a key. Instead the map or topic element addressed by the key name component of the
          @conkeyref is used in place of whatever map or topic element is addressed
        by the @conrefend attribute. See Using conrefend together with conkeyref for more information and for examples of this
        behavior.