What is ReferenceType in OPC UA?

Glossary · Address space

ReferenceType

In OPC UA, a ReferenceType is the NodeClass that defines the meaning of a reference between two nodes. Every reference is typed, directed and named by a ReferenceType node. The types form a hierarchy under References, split into HierarchicalReferences, which build browse trees, and NonHierarchicalReferences, which do not.

The hierarchy

References at i=31 is the abstract root. Beneath it HierarchicalReferences at i=33 covers Organizes, Aggregates - and its subtypes HasComponent, HasProperty, HasOrderedComponent - plus HasSubtype, HasEventSource and HasNotifier. NonHierarchicalReferences at i=32 covers HasTypeDefinition, HasModellingRule, HasEncoding, HasDescription, GeneratesEvent, HasInterface and the Part 17 alias references. Only hierarchical references contribute to the tree a browse client draws.

Symmetric, InverseName and direction

A ReferenceType node adds two attributes to the usual set. Symmetric says whether the reference means the same in both directions; if it does not, InverseName gives the LocalizedText for the reverse reading - HasComponent inverse is ComponentOf, HasSubtype inverse is SubtypeOf. Every reference exists in both directions in the address space model; whether a server will let you browse it backwards is another matter.

Using ReferenceTypeId as a browse filter

Browse takes a ReferenceTypeId plus an IncludeSubtypes flag. Browsing with HierarchicalReferences and IncludeSubtypes true gives you the tree; browsing with HasComponent only gives you composition and skips folder membership. Filtering here rather than client-side is the single biggest win when browsing a large PLC address space, and it avoids exhausting the server's continuation points.

Example

The reference type hierarchy, with the well-known numeric identifiers

References                        (i=31, abstract)
 +- HierarchicalReferences        (i=33, abstract)   builds the browse tree
 |   +- HasChild                  (i=34, abstract)
 |   |   +- Aggregates            (i=44, abstract)
 |   |   |   +- HasComponent      (i=47)   inverse: ComponentOf
 |   |   |   +- HasProperty       (i=46)   inverse: PropertyOf
 |   |   |   +- HasOrderedComponent
 |   |   +- HasSubtype            (i=45)   inverse: SubtypeOf
 |   +- Organizes                 (i=35)   inverse: OrganizedBy
 |   +- HasEventSource / HasNotifier
 +- NonHierarchicalReferences     (i=32, abstract)
     +- HasTypeDefinition         (i=40)   inverse: TypeDefinitionOf
     +- HasModellingRule / HasEncoding / HasDescription
     +- HasInterface / GeneratesEvent

The mistake people make: Treating the address space as a tree because the client draws one. References form a directed graph: a node can be reached from several parents, cycles are legal, and non-hierarchical references such as HasTypeDefinition jump sideways out of the tree entirely. Recursive browsers with no visited-set loop forever on the first server that Organizes a node into two folders.

Defined in: Part 3 - Address Space Model, ReferenceType NodeClass and the standard reference type hierarchy.

Related concepts

HasComponent HasProperty HasSubtype HasTypeDefinition Organizes Address space NodeClass

Status codes you will meet

BadReferenceTypeIdInvalid BadBrowseDirectionInvalid BadNoContinuationPoints

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.