This setting only appears when defining a Day schedule. Check this setting to create a so-called 24-hour schedule, that is a schedule that stretches over 24 hours. This type of schedule is sometimes used for machine recording, when recording can take place at any time during the day or night, and can also stretch over several days. The schedule is linked to the machine - i.e. the employee - in the Update Employee procedure.
When this setting is activated, the schedules start and end times are automatically set to 00:00 and 00:00 respectively, and the flex time settings are greyed out. You can still enter breaks just like a regular schedule.