This is applicable only when you're working with multiple fiscal years, say year 2007 and year 2008. At the end of the financial year 2007, the closing balances of all balance sheet accounts and the net profit/loss are carried forward to the next year 2008. This is a journal entry posted to carry forward journal on the 1st day of the new fiscal year, that get's created automatically when a Fiscal Calendar is created.
Now, as there may be some adjustments to be done to previous year's transactions, if you create/update/delete any of the previous year's journal entry, the final closing balances of those updated accounts will be different from that of the carry forward entry in the new year. In such a scenario, you can use this update carry forward entry process to correct the opening balances of the new year.
Hope this helps.
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Prashant