Aside from initiating a new CallBack from the New CallBack form, you can also initiate a CallBack to fill an open slot from within the Crew Scheduler.
Click an 'Open Slot'.
Then in the popup window, click 'Fill w/ CallBack'.
That will route you back to the normal CallBack form, however you will notice that is it now 'linked' to an assignment. In this case, 'Station 2'.
You'll notice that the Associate Shift is listed, "Station 2"; the open slot Start date and End Date are also pre-populated; and finally the # of 'Open Slots' is filled out. In this case, 3 Firefighters, due to there being 3 open slots for this assignment. You can edit this information, including Start / End date and times, as well as the number of positions needed. When finished, complete the rest of the form as usual and initiate the new CallBack.
Once an employee accepts the shift, they are automatically placed in the assignment. 'CB' text is added next to the employees name, indicating this shift was filled via a CallBack.