The term Referenceable Members is used in normative language for the OriginResources property in the EventDestination entity definition ... but the term does not appear to be defined in the Redfish Specification or elsewhere obvious. What precisely does it mean? Does it include the use of JSON pathing to reference information with a JSON document?
A Referenceable Member really is just a JSON object in a Resource that also has an @odata.id property. The reason for doing this is there are cases where we want to leverage using Navigation Properties in the data model so that we can show relationships with objects nested in a given resource. Because of this, we define Recerenceable Members as an EntityType in the CSDL schemas instead of the ComplexType definition; Navigation Properties must point to an EntityType with a valid @odata.id property.
For example, lets take an event in regards to a temperature sensor. Since the Thermal resource for a Chassis consists of a list of many Temperature sensors (which are defined as Referenceable Members within the Thermal resource), it wouldn't be very useful to simply give a link to the Thermal resource. Instead, the OriginOfCondition property for the Event payload can link to the individual Temperature object in the Thermal resource using the specific Temperature's @odata.id property.