Aside from initiating a new CallBack from the CallBack module, you can also initiate a CallBack to fill an open slot from within the CrewScheduler.
Click an 'Open Slot'. Then in the popup window, click the blue 'Fill w/ CallBack' button.
That will route you to the CallBack form, however you will notice that is it now 'linked' to an assignment. In this case, it is associated with the 'Detective' assignment on 06/26/2020.
You'll notice that the open slot Start date and End Date and times are also pre-populated. You can edit this information, including Start / End date and times, as well as the number of positions needed and which list you would like to hire back from. In this case, we are selecting to hire back for 1 position from the 'Detectives' list. Now go ahead and complete the rest of the form and initiate the new CallBack.