The reference documentation consists of the following sections:

Legal

Legal information.
Documentation Overview

About the Documentation, Getting Help, First Steps, and more.
Getting Started

Introducing Spring Boot, System Requirements, Servlet Containers, Installing Spring Boot, Developing Your First Spring Boot Application
Using Spring Boot

Build Systems, Structuring Your Code, Configuration, Spring Beans and Dependency Injection, DevTools, and more.
Spring Boot Features

Profiles, Logging, Security, Caching, Spring Integration, Testing, and more.
Spring Boot Actuator

Monitoring, Metrics, Auditing, and more.
Deploying Spring Boot Applications

Deploying to the Cloud, Installing as a Unix application.
Spring Boot CLI

Installing the CLI, Using the CLI, Configuring the CLI, and more.
Build Tool Plugins

Maven Plugin, Gradle Plugin, Antlib, and more.
“How-to” Guides

Application Development, Configuration, Embedded Servers, Data Access, and many more.

The reference documentation has the following appendices:

Application Properties

Common application properties that can be used to configure your application.
Configuration Metadata

Metadata used to describe configuration properties.
Auto-configuration Classes

Auto-configuration classes provided by Spring Boot.
Test Auto-configuration Annotations

Test-autoconfiguration annotations used to test slices of your application.
Executable Jars

Spring Boot’s executable jars, their launchers, and their format.
Dependency Versions

Details of the dependencies that are managed by Spring Boot.

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https://docs.spring.io/spring-boot/docs/2.4.0/reference/html/index.html