The Smart Client calls Application Server services mostly asynchronously. That means that a call to the Application Server does usually not block the user interface. If a single window is waiting for the response of the Application Server only that window will be blocked and you can still work with other windows.

Actions that are supposed to take a longer time will always be started as job. I.e. the creation of a report will be started as job on the Application Server and the Report window will never be blocked.