You can ensure a service is left unfilled by checking the "Leave this service unfilled (don't auto-schedule)" 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 "Leave this service unfilled (don't auto-schedule)"
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.