I too got the 4464c. As answered already. Normally means w-2s orn1099, WAGES in general. Example. I made a typo on my current 2019/2020 filing, I was off by one digit so the IRS does not have the same information as I have therefore I got selected by the computer for a human to review it and I am waiting and prepared if they ask to send them my amendment or my w2 but sometimes they fix that stuff on their own.
