3 Terms, definitions, and abbreviations

3.1 Document conventions

Throughout this document and the referenced other Parts of the series, certain document conventions are used. Italics are used to denote a defined term or definition that appears in the “Terms and definition” clause in one of the Parts of the series. Italics are also used to denote the name of a service input or output parameter or the name of a structure or element of a structure that are usually defined in tables. The italicized terms and names are also often written in camel-case (the practice of writing compound words or phrases in which the elements are joined without spaces, with each element's initial letter capitalized within the compound). For example the defined term is AddressSpace instead of Address Space. This makes it easier to understand that there is a single definition for AddressSpace, not separate definitions for Address and Space.

3.2 Terms and definitions

For the purposes of this document, the following terms apply.

3.2.1 AddressSpace

collection of information that an OPC UA Server makes visible to its Clients

Note 1 to entry: See Part 3 for a description of the contents and structure of the Server AddressSpace.

3.2.2 Aggregate

a function that calculates derived values from Raw data Note 1 to entry: Raw data may be from a historian or buffered real time data. Common Aggregates include averages over a given time range, minimum over a time range and maximum over a time range.

3.2.3 Alarm

type of Event associated with a state condition that typically requires acknowledgement

Note 1 to entry: See Part 9 for a description of Alarms.

3.2.4 Attribute

primitive characteristic of a Node

Note 1 to entry: All Attributes are defined by OPC UA, and may not be defined by Clients or Servers. Attributes are the only elements in the AddressSpace permitted to have data values.

3.2.5 Certificate

digitally signed data structure that describes capabilities of a Client or Server

3.2.6 Client

software application that sends Messages to OPC UA Servers conforming to the Services specified in this set of specifications

3.2.7 Condition

generic term that is an extension to an Event

Note 1 to entry: A Condition represents the conditions of a system or one of its components and always exists in some state.

3.2.8 Communication Stack

layered set of software modules between the application and the hardware that provides various functions to encode, encrypt and format a Message for sending, and to decode, decrypt and unpack a Message that was received

3.2.9 Complex Data

data that is composed of elements of more than one primitive data type, such as a structure

3.2.10 Discovery

process by which OPC UA Client obtains information about OPC UA Servers, including endpoint and security information

3.2.11 Event

generic term used to describe an occurrence of some significance within a system or system component

3.2.12 EventNotifier

special Attribute of a Node that signifies that a Client may subscribe to that particular Node to receive Notifications of Event occurrences

3.2.13 Information Model

organizational framework that defines, characterizes and relates information resources of a given system or set of systems.

Note 1 to entry: The core address space model supports the representation of Information Models in the AddressSpace. See Part 5 for a description of the base OPC UA Information Model.

3.2.14 Message

data unit conveyed between Client and Server that represents a specific Service request or response

3.2.15 Method

callable software function that is a component of an Object

3.2.16 MonitoredItem

Client-defined entity in the Server used to monitor Attributes or EventNotifiers for new values or Event occurrences and that generates Notifications for them

3.2.17 Node

fundamental component of an AddressSpace

3.2.18 NodeClass

class of a Node in an AddressSpace

Note 1 to entry: NodeClasses define the metadata for the components of the OPC UA Object Model. They also define constructs, such as Views, that are used to organize the AddressSpace.

3.2.19 Notification

generic term for data that announces the detection of an Event or of a changed Attribute value; Notifications are sent in NotificationMessages.

3.2.20 NotificationMessage

Message published from a Subscription that contains one or more Notifications

3.2.21 Object

Node that represents a physical or abstract element of a system

Note 1 to entry: Objects are modelled using the OPC UA Object Model. Systems, subsystems and devices are examples of Objects. An Object may be defined as an instance of an ObjectType.

3.2.22 Object Instance

synonym for Object

Note 1 to entry: Not all Objects are defined by ObjectTypes.

3.2.23 ObjectType

Node that represents the type definition for an Object

3.2.24 Profile

specific set of capabilities to which a Server may claim conformance.

Note 1 to entry: Each Server may claim conformance to more than one Profile Note 2 to entry: The set of capabilities are defined in Part 7

3.2.25 Program

executable Object that, when invoked, immediately returns a response to indicate that execution has started, and then returns intermediate and final results through Subscriptions identified by the Client during invocation

3.2.26 Reference

explicit relationship (a named pointer) from one Node to another

Note 1 to entry: The Node that contains the Reference is the source Node, and the referenced Node is the target Node. All References are defined by ReferenceTypes.

3.2.27 ReferenceType

Node that represents the type definition of a Reference

Note 1 to entry: The ReferenceType specifies the semantics of a Reference. The name of a ReferenceType identifies how source Nodes are related to target Nodes and generally reflects an operation between the two, such as “A Contains B”.

3.2.28 RootNode

beginning or top Node of a hierarchy

Note 1 to entry: The RootNode of the OPC UA AddressSpace is defined in Part 5.

3.2.29 Server

software application that implements and exposes the Services specified in this set of specifications

3.2.30 Service

Client-callable operation in an OPC UA Server

Note 1 to entry: Services are defined in Part 4. A Service is similar to a method call in a programming language or an operation in a Web services WSDL contract.

3.2.31 Service Set

group of related Services

3.2.32 Session

logical long-running connection between a Client and a Server. Note 1 to entry: A Session maintains state information between Service calls from the Client to the Server.

3.2.33 Subscription

Client-defined endpoint in the Server, used to return Notifications to the Client Note 1 to entry: ”Subscription” is a generic term that describes a set of Nodes selected by the Client (1) that the Server periodically monitors for the existence of some condition, and (2) for which the Server sends Notifications to the Client when the condition is detected.

3.2.34 Underlying System

Hardware or software platforms that exist as an independent entity. UA Applications are dependent on an entity’s existence in order to perform UA services. However, the entity is not dependent on UA Applications. Note 1 to entry: Hardware and software platforms include physical hardware, firmware, operating system, networking, non-UA applications, as well as other UA Applications. A Distributed Control System, PLC/Device, and UA Server are examples of an Underlying System.

3.2.35 Variable

Node that contains a value

3.2.36 View

specific subset of the AddressSpace that is of interest to the Client.

3.3 Abbreviations

A&E Alarms and Events API Application Programming Interface COM Component Object Model DA Data Access DCS Distributed Control System DX Data Exchange HDA Historical Data Access HMI Human-Machine Interface LDAP Lightweight Directory Access Protocol
MES Manufacturing Execution System OPC OPC Foundation (a non-profit industry association) PLC formerly an acronym for “OLE for Process Control”. No longer used. Programmable Logic Controller SCADA Supervisory Control And Data Acquisition
SOAP Simple Object Access Protocol
UA Unified Architecture UDDI Universal Description, Discovery and Integration
UML Unified Modelling Language WSDL Web Services Definition Language
XML Extensible Markup Language