Some tasks involve one or more sub-tasks. Deleting a task cannot undo sub-tasks already completed. For example, if importing and converting multiple files appears as a single task, deleting that task before it is completed stops the import and conversion process, but files already converted and imported remain on disk. If you want to cancel a task involving multiple sub-tasks, you can instead let the process complete and then use Undo to completely undo all sub-tasks, as available.