There are at least three different ways to display a second time zone, and confusing them is the quickest way to buy the wrong watch. The distinction that really matters is between GMT traveller, GMT office (or caller), and simple dual time: three mechanisms with opposite logic.
In the GMT traveller, the hour hand moves in one-hour increments without stopping the other hands. Those who physically cross time zones — the pilot, the frequent long-haul traveller — adjusts the local time and the GMT hand continues to show the reference. In the GMT office, instead, it's the 24-hour hand that moves independently: local time remains unchanged, and only the second time zone you want to keep an eye on is adjusted. The original Rolex GMT-Master I was in fact a traveller: the GMT hand was mechanically coupled to the hour wheel, so moving the local time also moved the GMT. Modern movements like the Sellita SW330-2 or Seiko's 4R34 calibre adopt the caller, which is more convenient for those working between offices in different time zones but don't physically change zones every day.
Dual time is the most basic version: a second hand, often red, indicates home time while you're abroad. Some versions allow you to hide it under the hour hand when not needed. It's not a true GMT complication: there's no 24-hour scale, no automatic AM/PM discrimination.
To read AM and PM on the second time zone you need either a 24-hour scale on the bezel — fixed like in the Tudor Black Bay Pro, or rotating like in city bezels — or a day/night disc or ring integrated into the dial. Without one of these elements, you never know if three o'clock GMT is morning or afternoon.
There are then alternative systems to the hand: the rotating disc of the Miyota 9075 with colored indicator, the peripheral minutes of certain avant-garde watches, the rotating barrel of the original Ulysse Nardin Freak that indicates the time without hands or crown, and jump-hour systems where the number jumps instantly. The latter require powerful return springs — in the Gerald Charles Maestro the hour disc weighs twenty-five times a normal hand — and the precision of the instant jump improves readability compared to gradual transit. GPS Solar systems like Astron's 5X83 calibre eliminate the problem at its root: they synchronize via satellite and set the time zone automatically, with a declared accuracy of ±15 seconds per month.
From my bench I often see customers buying a GMT caller thinking they have a traveller. The test is simple: if by turning the crown in position 2 the hours and GMT move together, it's a traveller. If the hours stay still, it's a caller.