Concepts

This section explains many of the concepts surrounding the Unity Package Manager functionality:

• Versions
• Manifests
• Registry
• Package Management
• Package states
• Package sources

Versions

Multiple versions of each package are available, marking changes to that package along its life cycle. Every time a developer updates the package, they give it a new version number. A change in package version tells you whether it contains a breaking change (major), a new backward-compatible functionality (minor), or bug fixes only (patch), following Semantic Versioning.

To view the list of versions available for a specific package, see Finding a specific version.

Manifests

There are two types of manifest files:

• Project manifests (manifest.json) These store information that the Package Manager needs to locate and load the right packages, including a list of packages and versions declared as dependencies.
• Package manifests (package.json). These store information about a specific package, and a list of packages and versions that the package requires.

Both files use JSON (JavaScript Object Notation) syntax.

Registry

Unity maintains a central registry of official packages that are available for distribution. A package registry stores package contents and information (metadata) on each package version. By default, all Projects use the official Unity package registry, but you can add additional registries to store and distribute private packages or stage packages in development.

Package Management

The Unity Package Manager is a tool that manages the entire package system. Its primary tasks include the following:

• It communicates with the Unity registry and any additional registries you specify.
• It reads your Project manifest and fetches package contents and metadata.
• It installs, upgrades, and uninstalls packages, whether they are dependencies of the Project or one of the installed packages.
• It enables and disables Unity’s built-in packages.
• It displays information about every version of every package.
• It resolves conflicts when the Project and its packages require more than one package version.

The Unity Package Manager installs samples, tools, and Assets on a per-Project basis, rather than installing them across all Projects for a specific machine or device. It uses a read-only global cache to store downloaded package metadata and contents. Once installed, Unity treats package Assets just like any other Asset in the Project, except that these Assets are immutable (read-only). You can only change Assets from Local and Embedded package sources.

Package states

States indicate where the package is in the development cycle:

State	Description
In Development	The package developer is still creating the package. Usually this corresponds to having the package embedded in the developer’s Project.
Preview	The package is ready to use and can appear in the package registry, but it is not ready for use in production. This stage is like a beta cycle.
Production-ready	The package is safe to use in production and the developer has published it to the package registry.
Verified	The package has undergone testing and has been verified to work safely with a specific version of Unity, and all other packages verified for that version.

The Package Manager window displays a tag that corresponds to some of these states. For more information, see Tags.

Package sources

Sources describe where the package came from:

Source	Description
Registry	The Unity Package Manager downloads most packages from a package registry into a global cache on your computer as you request them. These packages are immutable (read only), so you can use them in your Project, but you cannot modify them or change their package manifests.
Built-in	These packages allow you to enable or disable Unity features (for example, Terrain Physics, Animation, etc.). They are immutable (read only).
Embedded	Any package stored inside your Project folder is embedded. This source corresponds with the in development state because you typically put all the scripts, libraries, samples, and other Assets your new package needs in a folder under your Project folder when you begin development on a package.
Local	You can install a package from any folder on your computer (for example, if you have cloned a development repository locally).
Git	The Package Manager installs Git packages directly from a Git repository instead of from the registry server.

To edit the package manifest for a package, see Inspecting packages.

The Package Manager window displays a tag that corresponds to some of these sources. For more information, see Tags.