Code D refers to contributions to a 401(k) plan. Code AA refers to contributions to a Roth 401(k) plan. Now, that seems to mean that if you have code AA in box 12 you should also have code D as both are types of 401(k) plans. In the case where code D is marked but code AA is not, this means that the contribution was to a Traditional 401(k) plan rather than a Roth?