Assets may be related to other assets to expose different information like being physically connected or one asset utilizes another asset. In that case, References are used to expose such functionality. Typically, those References are non-hierarchical References. OPC 10000-23 defines several of those ReferenceTypes, Companion Specification may define additional ReferenceTypes. In Figure 18, some examples are given. X:Asset1 and X:Asset2 are related with the symmetric 0:PhysicallyConnectTo Reference. X:Subasset1.1 utilizes X:Subsubasset1.2.1 using the 0:Utilizes Reference.