> If you have union dues to be deducted, it is a formula to compute it against the total wage you make (or it is a flat deduction amount).

Except when it isn't. For example, some folks have multiple roles, covered by different unions, with different rules. Those rules are applied to different parts of their "total wage". I suspect that some wages don't count for 401(k). And so on.

None of these things necessarily have simple definitions and they interact in complex ways. Any "general model" is likely to be wrong and seemingly minor changes can have huge consequences.