And employers place many conditions on compensation: initial amount, salary increases, cost of living increases, when it is paid out, if some can be transferred to a 401k, if profit sharing is paid directly to a retirement plan, when and how bonuses are paid and how much and when commissions are earned. 
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Just the same as an employer places conditions on the company healthplan, some medical services and some medications are covered and some are not. Most healthplans differ in many ways, each of those difference are employers placing conditions on that particular form of "compensation."