The 'Check For Duration Overlap in Prior Records' Checkbox enables code generation to go back into the file and check earlier bookings for durations extending past the current starting point for the Display Listbox. 

This defaults to %TRUE

It will check one Day prior on the Day or BizDay grid for appointments extending past midnight.

For the Week, Month, and Year grids, it will go back "X number of Time Units" checking for duration.  Time units is Days or months depending on the grid selected.

The Default number of units is 7.