Definitions and Concepts of a System

A system is an arrangement of interacting elements organized to achieve one or more purposes. The elements of a system may include hardware, software, people, processes, information, facilities, natural objects, services, or organizations.

A system should be understood by what it does, how it behaves, what value it provides, and how it interacts with its environment.

System Definition

In Systems Engineering, a system is not merely a set of parts. A system exists when elements interact in a way that produces behavior, capability, or value that the individual elements cannot achieve alone.

A system has:

• A purpose
• A boundary
• Elements
• Interfaces
• Interactions
• Inputs
• Outputs
• Behavior
• Constraints
• Stakeholders
• An operating environment
• A life cycle

System Purpose

The purpose of a system explains why the system exists.

Purpose may be expressed as:

• A mission objective
• A business capability
• A user need
• A service outcome
• A performance goal
• A safety or regulatory function
• A societal or environmental objective

A clear system purpose helps teams define what is inside the system boundary, what is outside it, and what the system must accomplish.

System Elements

System elements are the constituent parts of a system. They may be physical, logical, human, organizational, informational, or procedural.

Examples of system elements include:

• Components
• Subsystems
• Software applications
• Databases
• Operators
• Maintainers
• Support equipment
• Facilities
• Procedures
• Interfaces
• Policies
• External services

System elements are important because system behavior depends not only on the elements themselves, but also on how they interact.

System Boundary

The system boundary defines what is considered part of the system of interest and what is considered external.

The boundary helps establish:

• Scope of responsibility
• Interfaces with external systems
• Ownership and accountability
• Verification and validation scope
• Operational assumptions
• Contractual or organizational boundaries

System boundaries are not always fixed. They may change depending on the level of analysis, acquisition strategy, operational scenario, or life-cycle stage.

System Environment

The system environment includes everything outside the system that can affect the system or be affected by it.

The environment may include:

• Users and operators
• Other systems
• Physical conditions
• Cyber environments
• Regulatory constraints
• Organizational processes
• Supply chains
• Social and political factors
• Maintenance and support infrastructure

A system that performs correctly in a laboratory may fail in the real environment if environmental assumptions are wrong.

Interfaces

An interface is a shared boundary or connection between system elements, or between the system and external systems.

Interfaces may involve:

• Physical connections
• Electrical signals
• Data exchanges
• Mechanical loads
• Human-machine interaction
• Organizational handoffs
• Procedural dependencies
• Environmental interactions

Interfaces are a major source of system risk. Many system failures occur not because individual elements fail, but because interfaces are incomplete, inconsistent, misunderstood, or poorly controlled.

Interactions

Interactions are the relationships and exchanges between system elements.

Interactions may be:

• Material
• Energy-based
• Informational
• Behavioral
• Human
• Organizational
• Temporal
• Logical

Understanding interactions is essential because system-level behavior emerges from them.

Emergent Behavior

Emergent behavior is behavior of the whole system that cannot be fully understood by examining individual elements in isolation.

Emergent behavior may be desirable, such as improved mission capability, or undesirable, such as instability, unsafe operation, cascading failures, or unexpected user behavior.

Systems Engineering seeks to identify, model, analyze, verify, and validate emergent behavior as early as practical.

Stakeholders

Stakeholders are individuals, groups, or organizations that have an interest in the system.

Stakeholders may include:

• Users
• Customers
• Operators
• Maintainers
• Owners
• Regulators
• Suppliers
• Developers
• Test teams
• Safety authorities
• Security authorities
• Business sponsors
• Communities affected by the system

Stakeholder needs are the foundation for system requirements and validation criteria.

System Requirements

System requirements define what the system must do, how well it must perform, and what constraints it must satisfy.

Requirements may address:

• Functionality
• Performance
• Interfaces
• Safety
• Security
• Reliability
• Availability
• Maintainability
• Usability
• Supportability
• Sustainability
• Regulatory compliance
• Physical constraints
• Environmental constraints

Good requirements are clear, necessary, feasible, verifiable, and traceable to stakeholder needs.

System Architecture

System architecture describes the fundamental organization of a system. It identifies major system elements, their relationships, interfaces, behaviors, and principles of design.

Architecture provides a bridge between requirements and implementation.

It helps answer:

• What are the major system elements?
• How are responsibilities allocated?
• How do elements interact?
• What interfaces must be controlled?
• What design decisions shape the system?
• How will the system evolve?

System Life Cycle

A system life cycle is the progression of a system from concept through retirement.

Typical life-cycle stages include:

• Concept
• Development
• Production
• Utilization
• Support
• Retirement

For more detail, see:

Life Cycle Stages

Key Takeaway

A system is defined by purpose, structure, behavior, interactions, and context. Systems Engineering focuses on the total system, not only its individual parts.