There is a special property type, UDO_REF, that can be used in both entities and user data objects. This property type makes it possible to create a connection between an entity and a user data object, or between two user data objects. This property type is a key feature of user data objects, as it allows the creation of complex graphs made up of different types of user data objects and entities that can be used by the entity scripts as desired. A UDO_REF property is nothing more than a reference to a user data object. When an entity or a user data object with a UDO_REF property is loaded, the user data object referenced by the UDO_REF property could exist in an unloaded state if the user data object referenced hasn't been loaded yet. In this case, the script will only be able to get the user data object's unique identification number through the guid attribute. Once the referenced user data object is loaded, all it's attributes and properties can be accessed.

The most important example of this property type is in the PatrolNode user data object. The old patrol path system, including the old PATROL_PATH property type, have been deprecated. Patrol functionality is now achieved with the PatrolNode user data object, which can be linked to other PatrolNode objects through an array of UDO_REF properties. Entities that wish to patrol through a graph of PatrolNode objects just need have a UDO_REF property that links to a PatrolNode.