Most generally passes were issued to employees, regulatory personel, favored officials of other roads and such. After so much time as an employee(in the crafts) you would be issued one that allowed for free travel on your home road(including rest of family, in most cases) and reduced rate of travel on other roads. Then there were the trip passes that had to be applied for each time one was desired.
Employees that had to ride trains as part of their work(other than Train and Engine service folks) might be issued one. Examples could be a signal maintainer, lineman, officials were suspossed to have them and there have been instances where the crew requested same and when none could be provided said official was asked to de-train. Passes for supervision were generally endorded "Good on head end, or good on locomotive" as extra folks were not susposed to be on the head end of a train. A very broad overview, hope it helps. Andy