May I know what is the different between currency differences and exchange rate gain/loss?

By right system shall auto calculate exchange rate gain/loss based on invoice date and payment date. Why should we using this session or when should we use this session?

Thanks

The 'calculate currency differences' session is used for accounts in the GL that are in a different currency than the Company's Home Currency. We don't use this session at my company. All accounts are in the Company's HC.
You will want to use 'Exchange Gain/Loss' to calculate the difference between currency at purchase to invoicing.
Hope this helps.
BB