The Unified Modeling Language (UML) is a standardized general-purpose modeling language managed by the Object Management Group (OMG). UML provides a set of graphic notation techniques to create visual models for software-intensive systems. In UML 2.2, there are 14 types of UML diagrams, divided into two categories:

7 diagram types represent structural information
Another 7 represent general UML diagram types for behavioral modeling, including four that represent different aspects of interactions.

These diagrams can be categorized hierarchically as shown in the following UML diagram map:

Structural Diagrams

1. Class Diagram

Purpose: Illustrates the static structure of a system by showing the system’s classes, their attributes, operations (or methods), and the relationships among objects.
Usage: Widely used in software engineering to model the static view of an application.

2. Component Diagram

Purpose: Depicts how components are wired together to form larger components or software systems.
Usage: Useful for high-level architecture design and understanding the organization of a system.

3. Deployment Diagram

Purpose: Shows the physical deployment of artifacts on nodes (physical resources).
Usage: Essential for understanding the hardware and software configuration of a system.

4. Object Diagram

Purpose: Represents a snapshot of the detailed state of a system at a point in time.
Usage: Often used to illustrate examples of data structures.

5. Package Diagram

Purpose: Organizes the elements of a model into groups to provide a better structure and manageability.
Usage: Helps in organizing large systems into manageable parts.

6. Composite Structure Diagram

Purpose: Shows the internal structure of a classifier, including its interaction points to other parts of the system.
Usage: Useful for modeling complex classifiers and their collaborations.

7. Profile Diagram

Purpose: Defines extensions to the UML language for specific platforms or domains.
Usage: Used to customize UML for particular needs, such as domain-specific modeling.

Behavioral Diagrams

1. Use Case Diagram

Purpose: Describes the functionality provided by a system in terms of actors and their goals (use cases).
Usage: Essential for requirements gathering and understanding user interactions.

2. Activity Diagram

Purpose: Models the dynamic aspects of a system by showing the flow of control from one activity to another.
Usage: Useful for business process modeling and workflow visualization.

3. State Machine Diagram

Purpose: Illustrates the states of an object and the transitions between those states.
Usage: Essential for modeling the lifecycle of an object.

4. Interaction Diagrams

Sequence Diagram: Shows how objects interact in a particular scenario of a use case.
Communication Diagram: Emphasizes the structure of the objects and their relationships.
Interaction Overview Diagram: Provides a high-level view of the flow of control in interactions.
Timing Diagram: Shows the interactions when a primary purpose of the diagram is to reason about time.

Is UML Huge and Complex?

UML is indeed a vast topic. It provides a large volume of diagramming notation grouped into 14 different UML diagram types, each serving different purposes and addressing different aspects of development needs.

Each UML diagram type provides a large set of constructs and notation that cover different needs for most software development projects.
The UML specification has more than 700 pages, and is often considered too complex, which can negatively impact its perception and adoption.
Typically, users tend to consider and use only a portion of its diagrams/constructs.

Learn the Most Essential UML Diagrams and Notation

Grady Booch, one of the most important developers of the Unified Modeling Language, stated that “For 80% of all software only 20% of UML is needed.”

What Does the UML Survey State?

We can interpret the results of the UML survey by assuming that if a diagram is:

Widely used if it is used in ≥ 60% of the sources
Scarcely used if it is used in ≤ 40% of the sources

UML Diagram Usage

UML Diagram	Usage Percentage
Class Diagram	100%
Component Diagram	80%
Deployment Diagram	80%
Object Diagram	71%
Package Diagram	70%
Communications Diagram	82%

Develop Your UML Learning Plan

Based on the figures collected in the survey, you can develop a meaningful UML learning plan. This article also provides an image map for quickly visiting a series of “What is UML Diagram” papers.

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Reference – What are the used UML diagrams? A Preliminary Survey, Gianna Reggio, Maurizio Leotta, Filippo Ricca, Diego Clerissi DIBRIS – Universit’a di Genova, Italy