Built-in packages allow users to toggle Unity features on or off through the Package Manager. Enabling or disabling a package reduces the run-time build size. For example, most Projects don’t use the legacy Particle System.
When you remove a built-in package of this feature, Unity does not include the related code and resources when you build your final application. Typically, these built-in packages contain only the package manifest and are bundled with Unity (rather than available on the package registry).
Click on any of the following links to see the documentation for that built-in package: