https://www.irs.gov/retirement-plans...erra-and-sscra
I’ve gone through this at a company without direct contribution, where we have to contribute to get a match.
After returning from long term mil, I got a letter from the company with an annual breakdown by years I was gone. As noted above, it’s based off USERRA-deemed compensation. It listed how much I needed to contribute to get the match, plus the max I could contribute up to the annual max for each “missed” year while I was on military. It also payed out the end date for making contributions (three times the leave length or five years, whichever was shorter, I believe).
For example, it was $18,000 in 2016. I could contribute $4,000 to get the full company match and another $14,000 to max it out (numbers are fictional). This is all in parallel with your current-year 401(k).
Make-up contributions filled the first eligible year bucket first. 2013, then 2014, etc. In 2016, when I got my W-2 for 2015, it listed Box 12, D codes by year to account for what was make-up contributed to each year:
D13 $8,000 (for 2013)
D14 $8,000 (for 2014)
D $10,000 (for 2017)
In 2017, my W-2 for 2016 had D14 for any remaining make up for 2014, D15 for 2015, etc. Had to file paper by hand since it was so rare the common tax software systems at the time couldn’t ingest D14 codes.
That’s way down in the weeds on details, but hopefully it helps.