Bootstrap is a complex front-end solution, which includes CSS, JavaScript, fonts and so on. In order to allow you the most flexible control over Bootstrap components, this extension provides several asset bundles. They are:
Particular application needs may require different bundle (or bundle combination) usage. If you only need CSS styles, yii\bootstrap4\BootstrapAsset will be enough for you. However, if you intend to use Bootstrap JavaScript, you will need to register yii\bootstrap4\BootstrapPluginAsset as well.
Tip: most of the widgets register yii\bootstrap4\BootstrapPluginAsset automatically.