Assets, based on their location, can be classified as:
- source assets: the asset files are located together with PHP source code which cannot be directly accessed via Web. In order to use source assets in a page, they should be copied to a Web directory and turned into the so-called published assets. This process is called asset publishing which will be described in detail shortly.
- published assets: the asset files are located in a Web directory and can thus be directly accessed via Web.
- external assets: the asset files are located on a Web server that is different from the one hosting your Web application.
Reference: Yii2 Doc