In many companies, it is common for employees to be assigned to on-call duties. During these times, they should of course no longer be available for other shifts. 

Often there is an additional regulation, how much time an employee is credited if he or she is not assigned.

To automate such a regulation as well as possible there is the following trick:

Decide for which shifts you need a standby or backup shift:

Create an appropriate backup shift:

It is important that you define a break in the shift. To do that, click on "Add break" and define the start and end of the break.
You just have to keep in mind that the remaining time will be the same as the payment period, if you don't need to use a standby. In the above example, the rule is to pay two hours.

Now you can assign an employee to a new shift.

If the employee has to step in as a backup, you can simply remove him from the backup shift and enter him in the corresponding shift.
Another possibility would be to adjust the times of the backup shift according to how the employee actually worked.