The "Plug and Play" is not describing the ESU. It is describing an NER within an ESU.
Plug and Play describes the way localization business functions are plugged into various programs. The program calls a plug-and-play NER which checks if a country localization is active for the user and if so, calls a country server function which finally calls the localization objects.
Take a look at Oracle support note "Localizations: Country Server Methodology (R96450) (Doc ID 2205125.2)" for full details.