I am confused about the use of the UTC/local indicators and the standard/wall indicators in a tzfile. According to tzfile(5), they "are used when a time zone file is used in handling POSIX-style time zone environment variables". Does this mean that these indicators are irrelevant when computing local time using only a tzfile (without looking at the environment).
That's correct: the ttisgmt and ttisstd indicators can be ignored if you're simply trying to figure out what wall clock time is for the zone that's associated with the file. (The indicators are there for the case when the time zone file is used as a "template" for the DST rules to be used when handling POSIX-style TZ environment variables; you need to know whether the transition time is defined in terms of UTC, local wall clock time, or local standard time to be able to know when translating the transition time for use in another zone.) --ado