OPC UA’s flexibility comes from its data modeling system, but that system introduces a vocabulary of its own — Nodes, Namespaces, Address Spaces, and the types that define them. These terms describe how an OPC UA server structures and exposes its information to clients.
| Term | Definition |
|---|---|
| Address Space | The hierarchical collection of Nodes an OPC UA server exposes to represent its data, structure, and behavior. |
| Namespace | A unique identifier (URI) that distinguishes Nodes defined by different vendors or specifications within an OPC UA Address Space. |
| NodeId | The unique identifier used to reference a specific Node within an OPC UA server’s Address Space. |
| Node Class | The category a Node belongs to, such as Object, Variable, or Method, which determines its allowed attributes. |
| Information Model | The structured definition of Object Types, Variable Types, and References an OPC UA server uses to represent real-world equipment and data. |
| Reference Type | Defines the kind of relationship between two Nodes in the Address Space, such as HasComponent or HasProperty. |
| Object Type | A template that defines the structure shared by all Object instances of that type. |
| Variable Type | A template defining the data type and constraints for Variable instances of that type. |
| BrowseName | The non-localized, machine-readable name used to identify a Node when browsing an Address Space. |
| DisplayName | The human-readable, often localized, name shown for a Node in client applications. |
| Attribute | A property built into every Node, such as its NodeId, BrowseName, or Value, that defines its basic characteristics. |
| Method | A callable function exposed by an OPC UA server that a client can invoke to trigger an action on the server or device. |
| Event | A notification raised by an OPC UA server to report a transient occurrence, such as a process condition or operator action. |
| Alarm | A persistent condition reported by an OPC UA server, typically requiring operator acknowledgment, such as a high-temperature warning. |
| Server | The OPC UA application that exposes process data and functionality from an underlying device or system to clients. |
| Client | The OPC UA application that connects to a server to read, write, subscribe to, or invoke functionality on its data. |
| Endpoint | The network address and security configuration a client uses to connect to a specific OPC UA server. |
| Profile | A defined set of OPC UA features and Conformance Units that a server or client implementation supports. |
| Conformance Unit | A discrete, testable set of OPC UA functionality used to certify that an implementation meets a specification’s requirements. |
| Browse | The client operation of querying a server’s Address Space to discover available Nodes and their references. |
| ProgID (OPC Classic) | A human-readable alias used to identify a specific COM-based OPC Classic server on a system. |
| In-Process / Out-of-Process (OPC Classic) | Describes whether a COM-based OPC server runs inside the client’s process as a DLL, which is faster but harder to share across multiple clients, or as a separate executable, which supports remote access and multiple simultaneous clients. |