Currency rate at date if purchase
Exchange rate for international stocks purchases
My portfolio holds US and European stocks. My main currency is the Euro and Yahoo Finance uses current EUR/USD exchange rates to estimate the current portfolio value, however, since there is no initial exchange rate when the stock is bought, the values tend to be inflated (the exchange rate is determined from the purchase date, but that can be different)
Alsgren Christian commented
I completely agree. The total returns shown are simply not correct in native currency anymore due to Yahoo Finance not counting the exchange rate at purchase correctly.