Why can't I use the same deduction codes for certain benefits?
Because deduction codes determine the way employees are taxed for their portion of their benefits, you cannot use the same deduction code for pre-tax and post-tax benefits.
For example, you can use the code Health for both Medical and Dental deductions, but you cannot use Retirement for both 401(k) and Roth 401(k), as these benefits are taken pre-tax and post-tax respectively.