Each pay code has the ability to filter who will be eligible to use it.  Eligibility can vary based on the many configuration options, but most use OrgLevels or EmpStatus. 

The pay codes for which an Employee is eligible are easily viewed from the Pay Code button located on the User Maintenances screen (lower right).  The Pay Code button opens the Pay Code Eligibility screen, and details about each pay code can be viewed by clicking the associated Eligibility Details button.

It should be noted that any changes made to a pay code accessed this way will affect everyone who uses it.