The plan document ALWAYS matters because it is the controlling legal document that allows the deferrals to happen, and plan documents can be written differently by every company and violating your written plan could carry big penalties. So when an employee signs up they are agreeing to have the amount withheld from whatever the plan says their eligible compensation is. So maybe the plan document says vacation pay is not considered compensation for deferral purposes, or maybe the plan document says no deferrals will be taken from any paychecks issued after termination. Rule#1 with 401ks is follow your plan document.
i guess i was under the assumption that the IRS actually set some kind of guidelines in the actual 401k irs code.