OPC UA Glossary

Key definitions for industrial interoperability, modeling and architecture

OPC UA now plays a central role in industrial communication, semantic modeling and interoperable system architecture. This glossary explains the key terms used in real OPC UA projects, from AddressSpace and NodeSet to PubSub, UAFX and security models.
It is written for engineers, system architects, automation specialists, developers and technical decision-makers working on industrial OPC UA systems.

Core OPC UA Concepts

AddressSpace
The structured representation of all data exposed by an OPC UA server. It contains objects, variables, methods and events organized in a typed and navigable hierarchy.

Node
The fundamental element of the AddressSpace. A Node represents either a type (model) or an instance (runtime object), and contains attributes and references to other nodes.

NodeId
A unique identifier assigned to each Node. It combines a namespace and an identifier and is used by all OPC UA services.

Namespace
A logical container used to separate standard, vendor-specific and application-specific models. Defined by a URI and an index.

Reference
A typed relationship between two nodes, defining hierarchy, composition or semantic links.

Information Modeling

Object
An instance representing a physical or logical component. It can contain variables, methods and events.

Variable
A node that exposes a value such as a measurement, state or parameter. It can be read and optionally written.

Method
A callable function exposed by the server, allowing clients to trigger actions (e.g. Start, Reset, Calibrate).

DataType
Defines the structure and format of data. OPC UA supports primitive types, arrays and complex structures via ExtensionObject.

Type System
The modeling foundation of OPC UA, allowing the definition of ObjectTypes, VariableTypes, DataTypes, ReferenceTypes and EventTypes.

NodeSet (UANodeSet XML)
A standardized XML format used to define OPC UA information models. It enables model exchange, versioning and automated generation of AddressSpaces.

Communication and Runtime

Client OPC UA
An application that connects to a server to read, write, subscribe to data or call methods. Typical examples include SCADA, MES or digital twins.

OPC UA Server
An application that exposes data, methods and events through an OPC UA information model.

Endpoint
The access point of a server, including protocol, security mode and configuration. Example: opc.tcp://host:4840

Session
A logical context between client and server that manages identity, security and communication parameters.

Subscription
A mechanism that allows a client to receive automatic updates when monitored data changes.

MonitoredItem
An element within a subscription that tracks a variable or event and triggers notifications.

DataChange Notification
A message sent when a monitored value changes according to defined criteria.

Event
A structured notification generated by the server to signal alarms, state changes or conditions.

Security

SecureChannel
A secure communication layer ensuring message integrity, confidentiality and authentication.

Security Policy
A set of cryptographic algorithms used for signing and encryption (e.g. Basic256Sha256, Aes256-Sha256-RsaPss).

Certificate / PKI
X.509 certificates used to authenticate applications and secure communication. OPC UA relies on a Public Key Infrastructure.

User Authentication
Mechanisms used to authenticate users:

  • Anonymous
  • Username/Password
  • X.509 Certificate
  • Token-based (e.g. OAuth2 via gateways)

PubSub and Data Distribution

PubSub (Publish/Subscribe)
A communication model where publishers send messages without knowing recipients, and subscribers receive selected data. Supports UDP, MQTT and TSN.

Publisher
Component that sends data messages derived from the AddressSpace.

Subscriber
Component that receives and processes published data.

Broker / Brokerless
PubSub can operate through a broker (e.g. MQTT) or directly via UDP multicast.

UAFX and Industrial Communication

UAFX (OPC UA Field eXchange)
An extension of OPC UA enabling deterministic communication at field level, based on structured information models.

AutomationComponent
A UAFX concept representing an industrial device with its data model, capabilities and diagnostics.

FunctionalEntity
A modular sub-model within an AutomationComponent defining process functions and interfaces.

C2C / C2D / D2D
UAFX communication patterns:

  • Controller-to-Controller
  • Controller-to-Device
  • Device-to-Device

Additional Concepts

Discovery Server (LDS / LDS-ME)
A service used to register and discover OPC UA servers on a network.

ExtensionObject
A container used to transport complex structured data.

Historian
A system that stores and provides access to historical OPC UA data.

TSN (Time-Sensitive Networking)
A set of Ethernet standards enabling deterministic communication, used in UAFX.

Companion Specification
Domain-specific models built on OPC UA to represent industry standards (e.g. ISA-95, Euromap).

OpenOpcUa

OpenOpcUa
An industrial OPC UA technology initiative built around a complete C/C++ implementation and middleware architecture. It enables model-driven engineering, cross-platform deployment and strict alignment between NodeSet-based models and runtime execution.