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ICS networks are very heterogeneous environments. There are many software/hardware platforms, applications and protocols present in these environments. Because of this, ATT&CK for ICS techniques don't necessarily apply to all ICS assets. ATT&CK for ICS adds the organizational unit of Assets to aid ATT&CK for ICS users to understand which techniques are applicable to their environment.

Here we list the asset categories that are commonly found in industrial control systems. Assets encompass everything from Windows workstations running specialized applications to embedded devices with analog inputs and outputs. Each page details the functions, Levels and Techniques associated with the asset category.

Control ServerA device which acts as both a server and controller, that hosts the control software used in communicating with lower-level control devices in an ICS network (e.g. Remote Terminal Units (RTUs) and Programmable Logic Controllers (PLCs)).1
Data HistorianA centralized database located on a computer installed in the control system DMZ supporting external corporate user data access for archival and analysis using statistical process control and other techniques.2
Engineering WorkstationThe engineering workstation is usually a high-end very reliable computing platform designed for configuration, maintenance and diagnostics of the control system applications and other control system equipment. The system is usually made up of redundant hard disk drives, high speed network interface, reliable CPUs, performance graphics hardware, and applications that provide configuration and monitoring tools to perform control system application development, compilation and distribution of system modifications.2
Field Controller/RTU/PLC/IEDController terminology depends on the type of system they are associated with. They provide typical processing capabilities. Controllers, sometimes referred to as Remote Terminal Units (RTU) and Programmable Logic Controllers (PLC), are computerized control units that are typically rack or panel mounted with modular processing and interface cards. The units are collocated with the process equipment and interface through input and output modules to the various sensors and controlled devices. Most utilize a programmable logic-based application that provides scanning and writing of data to and from the IO interface modules and communicates with the control system network via various communications methods, including serial and network communications.2
Human-Machine InterfaceIn computer science and human-computer interaction, the Human-Machine Interface (HMI) refers to the graphical, textual and auditory information the program presents to the user (operator) using computer monitors and audio subsystems, and the control sequences (such as keystrokes with the computer keyboard, movements of the computer mouse, and selections with the touchscreen) the user employs to control the program. Currently the following types of HMI are the most common:

Graphical user interfaces(GUI) accept input via devices such as computer keyboard and mouse and provide articulated graphical output on the computer monitor.

Web-based user interfaces accept input and provide output by generating web pages which are transported via the network and viewed by the user using a web browser program. The operations user must be able to control the system and assess the state of the system. Each control system vendor provides a unique look-and-feel to their basic HMI applications. An older, not gender-neutral version of the term is man-machine interface (MMI).

The system may expose several user interfaces to serve different kinds of users. User interface screens may be optimized to provide the appropriate information and control interface to operations users, engineering users and management users.2
Input/Output ServerThe Input/Output (I/O) server provides the interface between the control system LAN applications and the field equipment monitored and controlled by the control system applications. The I/O server, sometimes referred to as a Front-End Processor (FEP) or Data Acquisition Server (DAS), converts the control system application data into packets that are transmitted over various types of communications media to the end device locations. The I/O server also converts data received from the various end devices over different communications mediums into data formatted to communicate with the control system networked applications.2
Safety Instrumented System/Protection RelayA safety instrumented system (SIS) takes automated action to keep a plant in a safe state, or to put it into a safe state, when abnormal conditions are present. The SIS may implement a single function or multiple functions to protect against various process hazards in your plant.3 The function of protective relaying is to cause the prompt removal from service of an element of a power system when it suffers a short circuit or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system.4