From the link below, we can see:
In Domino 7 and 8 releases, the request from Notes to compact a database was changed. This functionality is now handled by the Update (updall) task so that the connection to the Domino server is not tied up waiting for a mail file to be compacted. The Update task places the Compact request in a pending queue (immediate) and processes it when it has free cycles.
If end user click "Compact" button of a huge database, it may impact server performance. To stop the compact task, we can issue command "tell update quit" to stop the compact processed by Update task. But "tell update quit" will stop all Update tasks loaded.
Customer would like to have the option to stop Compact handled by Update without stop all Update tasks loaded.