How are benefit plan types attached to the tax tables?

We are trying to understand how a different group of associates are being taxed differently on the same benefits - example if basic life insurance.  One group is being taxed for their basic life with the $50,000 employer gift tax and then taxed on the remaining, however the other group of partners are not having taxes withheld.  Where does this calculate - is it the payroll COBOL or a setup table in Ben Admin?  Thank you!