Exchange rate picking in commitment item in Budget

Hi

I want to know on what basis system will pick exchange rate in commitment item in budget report. I have created PR.I have maintained necessary configuration settings (exchange rate for budget profile,planning profile) but still when i am creating import PR, system is not picking my required exchange rate instead of this, system is picking some other exchange rate for commitment rate calculation .may i know for PR commitment on what basis system will pick exchange rate?

Thanks in advance

Regards

Haran.