Form 2210 is for the underpayment of estimated tax penalty and is not always required, but if the underpayment is because of uneven income during the year then then the 2210 form is used to show that more income was in one quarter that other quarters of the year that will reduce the penalty, that is actualizing the income.
https://www.irs.gov/pub/irs-pdf/f2210.pdf
https://www.irs.gov/pub/irs-pdf/i2210.pdf
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
