You can ensure a service is left unfilled by checking the "Don't fill in this service when the auto scheduler is run" setting in the Edit Service dialog in the Schedule Skeleton.
- Open the schedule and select the service you wish to leave unfilled
- Click the "Edit Service..." button
- In the dialog box that appears, click the setting that says "Don't fill in this service when the auto scheduler is run"
When you Commence Scheduling, MSP will leave the service unfilled. You can then manually schedule these positions later or post them to the Web Terminal to allow volunteers to sign-up online.