Reports based on MRP Data,Online Data

Hi ,

We have some reports in which selection Options is :- a) MRP Data or b) MD04 Data

When MD04 is selected, report is calculated based on data from MD04 screen while MRP data is choosen, Report is selected from MD04 screen. I would like to know why this option is given. How program and from where program fetches data when MRP Data is selected and when MD04 data is selected. Thanks in advance.