Simply put it means that a previous determination (i.e. that you were entitled to or not entitled to unemployment benefits) is being withdrawn (i.e. treated as though it never happened). We would need to see which determination the letter you sent relates (or you can call them and ask a representative who works in that dept. who has familiarized himself or herself with your matter). It unfortunately also sounds like there will be some significant agency activity to correct / rectify all of the confusion and make this matter consistent with the law.
If you worked while certifying that you could not work, the agency will consider that to be fraud.
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