The offset - indicates that Pacific Time is obtained from GMT by adding -, that is, by subtracting 8 hours.
The offset differs in the summer because of daylight savings time, or DST.
This is a pure virtual API, so it is implemented in the concrete subclasses of Time Zone.
Time zone data changes often in response to governments around the world changing their local rules and the areas where they apply.
Several update strategies are possible, depending on the ICU version and configuration.