Since there is an overall limit on the amount that is permitted to be in box 3, it's better to calculate the difference between box 1 and box 5.
The year designator is to be used for a makeup contribution for a prior year under the Uniformed Services Employment and Reemployment Rights Act of 1994 (USERRA) after return from military service, although some employers mistakenly use the year indicator in conjunction with contributions for the current year.
