This feature was introduced in VuFind 5.0.
Cover layers are used by the dynamic cover generator to construct image layers (i.e. background and text).
If you want to change the way cover images are drawn, you can build your own rules in a plugin.
Default Namespace: \VuFind\Cover\Layer
Interface: \VuFind\Cover\Layer\LayerInterface
Service Locator Configuration Section in module.config.php: ['vufind']['plugin_managers']['cover_layer']
Service Manager Name for Service Locator: VuFind\Cover\Layer\PluginManager
See the General Plugin Information page for more details on VuFind plugins.