As mentioned before, if both are filled, pn-begda and pn-endda will hold the value for data selection period. pn-begps and pn-endps will hold the values for person selection period. You can use whichever date parameter you want in the program depending on your requirement.
As far as data retrieval is concerned, when data selection period is filled, it will pick those records that are valid for that date period and when person selection period is also filled, the records are further filtered such that each personnel number that is selected is also valid for the person selection period (i.e. person is valid in P0001 for at least one day in that period).
The data might be valid for a date, but the person may not be active on that date, hence such records will also be removed. For example, if an employee is terminated in the middle of the month, say 10th, he will have data records (e.g. his salary data) till end of month. So data will be retrieved for data selection period given between 15 and 30. However, since the employee is not valid on these days in P0001, no data will be selected for the same inputs in person selection period.