After ULC R1.3 onwards all ULC server applications interpret a connection request with the application name EXIT to mean that the server itself should exit. Hooks are provided in Smalltalk for the application to override the default behavior which is to do a System.exit immediately. It is up to the application developer to decide what actions should be performed when this EXIT request is received.
For example if you have started a ULC application on your local machine as a server on port 4444. Starting the UI Engine with the parameter -url ulc://localhost:4444/EXIT will cause the ULC application server to exit immediately.
To change the default behavior, in Smalltalk the developer must register for the event using the ulcWhenAboutToTerminateSend: API in UlcSystem.