Logic of currency exchange rate

Hi Team,

User is entering necessary voucher information from 3rd party application. In that they need the functionality to override currency exchange rate in case of currency other than USD. I come across table F0015 where these rates are getting stored.

Can you please guide me how system automatically determined the currency exchange rate? What basis it fetches out the conversion rate? GL date or invoice date or anything else?

Thanks

Sachin Modi