Expense Report Template in AP

When we setup expense report template and select Expense Category as Mileage or Per Diem for expense item the one field is there in the same form called 'Policy Schedule' which become mandatory field (Yellow color) and it is Lis of value and nothing is populating in this LOV. And it happeing only when we choose category as Mileage or Per Diem for other category it is not mandatory.

We have not setup and not using Iexpense module for expense report , only Core AP we are using for this. Could you suggest what this 'Policy Schedule' means and what to do with this, and how we can make it as normal field i.e. not mandatory.