When the package is ready for testing, the Unity package developer makes sure there is some basic test coverage, and that the package passes internal testing. The package also needs at least preliminary documentation, an updated changelog, and licensing.
By default, pre-release packages do not appear in the list of packages you can install; however, you can force the Package Manager to show them by enabling the Enable Pre-release Packages project setting.
Note: Prior to Unity Editor version 2021.1, the Package Manager used the “Preview” state to describe packages that are experimental or risky but otherwise mature, as well as packages that have not yet been fully validated as safe to use in production. Starting with 2021.1, the “Preview” state no longer exists, and packages can either be “Experimental” or “Pre-release”. This provides a clearer distinction between packages that are mature but risky to use, and packages that are almost fully mature.
Pre-release packages are still being verified to be safe to use with the current version of Unity, so be aware that any package that appears with the indicator is supported by Unity and guaranteed to be fully validated for use in production by the end of the LTS version of the current cycle. For more information about the pre-release state and the lifecycle of packages, see Package states and lifecycle.
For a list of stable packages released for this version of the Editor, see Released packages.