Currency rates can be imported from a currency rate provider, e.g. ECB. The currency rates imported are all against EUR (EUR-USD, EUR-GBP, EUR-CAD).

When a legal entity use another accounting currency than EUR e.g. GBP and post transactions with a transaction currency different from EUR e.g. CAD, D365 need a currency rate for GBP-CAD before the transaction can be posted. 

The currency pair and currency rates can be created and maintained manually. Often the customer want to automate the ECB import and update of the additional currency pair rates and a customization are made. 

I prefer that the calculation of the additional currency pair rates can be configured and run automatic based on the currency rates imported from ECB.

I don't know if that update can be done by exporting the ECB rates as a recurring job from the Data management workspace, have the calculation of additional currency pairs done automatic outside D365 and have a recurrent job import and update the exchange rate type selected on Ledger automatic, but this solution are less user friendly.