Mark as volume is a feature designed for shifts that don't need a specific end time. If you have an employee that can leave whenever their work is done, you would mark their shift as volume.
You will find this option when editing a shift you've created. You will need to set a Shift End Time before you mark as volume, so that the system can estimate hours for you.
When you publish the schedule, you can choose whether or not to Display Out Times for Shifts Marked as "Volume". When you opt out of this option, the end time for the shift will say "Volume" when employees receive their schedule. If you choose this option, the end time you estimated will show on their schedule, i.e. 10:30pm (v).