How do we report retirement plan contributions on an employee W2?

To learn how to properly report retirement contributions on your employee W2s, please refer to the following resource from the IRS:
See the Instructions for Forms W-2 and W-3 PDF for a complete list of codes.. Form W-2, Box 13. The "Retirement plan" indicator in Box 13 shows whether an employee is an active participant in your company's plan. If this box is checked, it lets the recipient know that depending on their filing status and modified adjusted gross income, they may not be entitled to a full deduction for their ...
What gets reported on the W2 depends on the type of plan and the type of contribution that is made. Employer contributions are not included, but box 13 is marked if the employee participates in the retirement plan.

The Unitarian Universalist (UU) Organizations Retirement Plan is an IRS qualified 401(a)/401(k) defined contribution, multiple employer, church retirement plan. This is not a 403(b) plan.

The Letter Code D in box 12a is used for EMPLOYEE ELECTIVE contributions to the plan.