From the employee website:
Nonrevenue and Reduced-Rate Travel Guidelines
The travelers’ overall appearance should be well-groomed, neat, clean, safe and respectful, from head to toe. Simply said, if the attire is appropriate for a revenue passenger to wear, then a nonrevenue passenger can wear the same attire.
Appropriate (all classes of travel) Not appropriate (all classes of travel)
Overall appearance should be well-groomed, neat, clean, safe and respectful, from head to toe.
Clothing should be respectful of fellow passengers.
Footwear – shoes are required unless the pass rider is not able to wear footwear due to a disability or physical condition.
Passenger that is (or appears to be) intoxicated
Passenger whose dress violates public decency laws and or community standards (examples include clothing that is sheer or inappropriately revealing or is designated as sleepwear, underwear, or swim attire)
Bare feet
Clothing that is excessively dirty, stained or torn
Clothing that is vulgar, offensive or suggestive
A few examples of relaxed attire which are permitted in all classes of travel:
Shorts
T-shirts
Jeans
Flip Flops
Caps