In Locale prefs, the list at the right-hand side of the Time Zone tab includes an arbitrary mixture of cities, regions and time zone names. The only clue to what the contents are supposed to be is that they come from an include file called cities.h that was added with the commit note "Added city selection for timezones".
Additionally, clicking on the "USA Pacific Daylight (PDT)" entry highlights the "Mountain time" area on the map (at least when I tried it in December :-) ).