Some asset types can have their own dependencies, much like JavaScript modules can. It's common to treat CSS and SVGs as assets, but both formats can include references to images, fonts or stylesheets. When processing assets to add hashes to filenames, it's important that changing the URL hash of a subresource also changes the hashes of any assets referencing it.

body {
  /* an image "dependency": */
  background-image: url(background.jpg);
}

When bundling the above CSS and its referenced image with hashed URLs, any changes to background.jpg will also change its hash (background.a1b2c.jpg). Since the CSS must be updated to reference the image's new URL hash, the CSS file must also be given a new hashed URL now that its contents have changed.