If we provide a currency in a G/L account, then you cannot post transactions into this account in any other currencies. we can only post the transactions related to that particular currency which is specified in the G/L account (COA).
For Example, when we select a currency in the journal, the system will list the accounts (that have currency in the account) in the journal line. when you select any other currency and try to add this account in the journal line the system will not lists this account as we have specified the currency in the account.
If you haven’t specified any currency in the G/L record, then the account is available for all currencies.