Account currency is that the GL account in which currency do you want to maintain. If you decided that you want to maintain in company code currency, you can post any currency on that account. If not, you want to maintain separate currency for that GL then exchange rate difference will come because of the conversion rate.
Stability in local currency – any GL account can’t be maintained on free item foundation and can’t in foreign currency similar to emptying account and discount account etc., while so event you can allow this notice to determine the stability in local currency.