Since the OnePoint Diagnostics Service scans contain some heavy steps, constraints have been imposed on simultaneous execution. When selecting immediate execution via the "Now" option, the task is only accepted when no other (similar) tasks are already executing on the target OnePoint Diagnostics server. Tasks already executing also include tasks started by some other OnePoint Diagnostics user.
When you prepare a task for execution in the future, you are prompted for confirmation when already a task is executing or when already some other task is pending for future execution. By selecting the "Yes" button of the pop-up message, you allow the task to be prepared (for future execution) even if task execution would result in parallel executing tasks. In other words, you have full control over the amount of active tasks in the future: it is your responsibility.