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).
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