The conkeyref attribute
For content references from map elements to map elements or topic elements to topic elements, the value of the conkeyref attribute is a key name, where the key must be bound to a map element (for references from map elements) or a topic element (for references from topic elements). For all other elements, the value of the conkeyref attribute is a key name, an optional solidus ("/"), and the ID of the target element, where the key name must be bound to the map or topic that contains the topic 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 content reference relationship. If no conref attribute is specified there is no content reference relationship. Processors should issue a warning when a conkeyref reference cannot be resolved and there is no conref attribute to use as a fallback. Processors may issue a warning when a conkeyref cannot be resolved to an element and a specified conref is used as a fallback.
The conrefend attribute, which defines the end of a conref range, may not 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.