The field is used only for flexible time schedules, where Format is Time From Sign In.
The field can be used to define what will happen with the time on the line.
Marking means the calculation will not push the time intervals. When calculating, the breaks must be fixed, thereby not affect the job in the fixed time interval.
The field must be blank, when Format Time of Day in lines Normal time and possibly Overtime is registered.
See also here.
See also
Reference
Time Schedule Lines
Schedule Type Override
This help includes both Dynamics NAV and Dynamics Business Central (BC). All references to NAV applies to both.