...
The Default Currency field is populated with the accounting currency that was selected on the Administration > Settings > Money form. If needed, you can select another currency as the currency of cash account. If the required currency is not in the list, add it to the list of available currencies. For details, refer to Currencies.
The Cash Account (create) form
...
In the GL Account Edit dialog box, in the GL Account field, you can view the default GL account assigned automatically to the created cash account.
The GL Account Edit form (for a cash account)
...
To edit the account number or other properties, click the 
button located to the right of the field. Note. you cannot change the section of the GL Account.
...