How does the clock keep them from clocking in when they are not supposed to?

How does it know everyone's schedule?

How does the clock know if an employee was approved to have time off?   

In most cases, employees can clock in or have someone clock in for them whenever they want. How does the clock keep them from clocking in when they are not supposed to? How does it know everyone's schedule? How does the clock know if an employee was approved to have time off? When I prepared payroll, I would not process the pay for any employee unless their supervisor approved it. Otherwise, they could work when they wanted and not work when they didn't want to and leave the company short-handed.