CAL is the abbreviation for “Configurator Abstraction Layer”. CAL comes with a standardized interface for configurators. With CAL you can develop an application that can talk to multiple configurators.
Mainly SAP configurators are supported. SAP configurators come with many different interfaces. If you want to use your application with two different configurators without CAL, you will need to adapt your application. With CAL this is not necessary.
CAL also extends configurators with missing features. Configurator specific parts can be loaded dynamically like “drivers”. This allows a hot-swap of versions or configurators without shutting down the server.