The code 1 is a normal early distribution distribution that unless you rolled it into an IRA is taxable as ordinary income and also subject to an additional 10% penalty.
The code 1M means that you had an outstanding loan and that money was taken to pay the loan and is also taxable just as the code 1 above, except the "M" means that you have until Oct 15, 2021 (assuming that it is a 2020 1099-R and you either filed for an extension OR filed your tax return by May 17, 2021), to roll it into a Traditional IRA by replacing the box 1 money with other funds, then is is a non-taxable rollover.
**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.**
