A system callback is a scheduled callback which the system will automatically dial on the date that is set. If the lead answers when the system calls them back, the call will be transferred to the next available agent. 

Note: The system will not dial a lead with a system callback schedule if there is no agent logged in and on ready status.

During the call, the Call Function buttons will appear at the top right of the agent page.  

Click the hangup button to end the call.

In the Disposition Window, select Call Back status and the Callback’s Time Section will appear at the bottom of the window.

Click the Callback's Time Zone option to select different time zones.

Click on the Callback's Time to open the calendar window. Click on the month to view all the months or use the arrow left and right to go next or the previous month.

Once the month is selected, you can now pick the date of the callback.

Once the day has been selected select the hour and minute of the callback. 

 In Callback Type, Select System Callback

Click on the Save changes button to save the settings.

As for the sample callback above, the Callback's Time Zone is Lead's Time Zone (Lead's time zone is EST) and the callback time is July 25, 2019, at 11:30 AM.

The system will calculate the time difference between the Agent's Time Zone (Configured on the Agent Callback Options) and the Lead's time zone.

Let's say that the Agent's Time Zone is on PST and the lead is on EST. The schedule callback is  July 25, 2019, at 11:30 AM.

See this link for more information about Personal Callback
How to Create a Personal Callback