本节介绍有关 Unity Package Manager 功能的许多概念：
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), 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.
manifest.json) 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.json) store information about a specific package, and a list of packages and versions that the package requires.
In the domain of Unity’s Package Manager, a package registry is a server that stores package contents and information (metadata) on each package version. Unity maintains a central registry of official packages that are available for distribution. By default, all projects use the official Unity package registry, but you can add additional registries to store and distribute private packages or stage custom packages while you are developing them.
Unity Package Manager 是管理整个包系统的工具。此工具的主要任务包括：
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 global cache to store downloaded package metadata and contents. Once installed in a project, Unity treats package assets just like any other asset in the project, except that these assets are stored inside the package folder and are immutable. You can only permanently change content from Local and Embedded package sources.
|注册表||The Unity Package Manager downloads most packages from a package registry server into a global cache on your computer as you request them. These packages are immutable, so you can use them in your project, but you cannot modify them or change their package manifests.|
|内置||这些包允许您启用或禁用 Unity 功能（例如，地形物理 (Terrain Physics)、动画 (Animation) 等）。这些包是不可变的。有关更多信息，请参阅内置包。|
|嵌入式||Any package stored inside your project folder is embedded. This source corresponds with the Custom 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 custom package.|
|本地||You can install a package from any folder on your computer (for example, if you have cloned a development repository locally).|
|Tarball (local)||You can install a package from a tarball file on your computer. The Package Manager extracts the package from the tarball and stores it in the cache. However, these packages are immutable, unlike installations from a local folder.|
|Git||The Package Manager installs Git-based packages directly from a Git repository instead of from the package registry server.|
The Package Manager window displays a tag that corresponds to some of these sources. For more information, see Tags.