The Person day setting (in hours) help you to determine the day calculation. This is used to calculate the Duration of the tasks.
For example, if your 1 Person-Day=8, then this number is used to calculate your task duration which will then be 1day.
If you have multiple work calendars with different work timings, we suggest that you set this value to the maximum work hours of the calendars i.e Work Calendar 1 has 8 hours working and Work Calendar 2 has 9 hours working, then the 1 Person Day setting should be set to 9.