When Period 12 is ready to be closed, the usual end of period process can be run by an Advanced user or System Administrator. This will do all the usual end of period processing, as well as create a special journal to close Profit and Loss accounts to the specified retained earnings account. Refer to End of Period Processing for the general steps on closing a period.
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The Retained Earnings journal:
Will close the balance of all the Profit and Loss accounts to the Retained Earnings account linked in System > Administration > Special Accounts
Be dated the last day of Period 12
Is a special journal type (RET), which will be excluded from most reports