This is a feature you would use, if you are using a fixed conversion rate for your currencies.
You can add your own fixed currency exchange rate and various date ranges to your fixed currency exchange rates. E.g. you might want to use one conversion rate for 1st of January - 1st of June and then another conversion rate from the 1st of June - 31st December.
How to add fixed currency exchange rate
Select "Products" > "Price catalog".
Select "Actions" and "Currency settings" from the drop down list.
Select "Add currency setting" to create a new fixed currency exchange rule.
Fill in the form to define the parameters for the currency exchange rate
Save and remember to publish your price catalogue to activate the new currency exchange rate.