When do you set the local altimeter under ICAO rules during descent or arrival?

Setting the local altimeter at the transition level (TLv) is important because it ensures that the aircraft is operating on the correct reference for altitude. The transition level represents the lowest flight level above the transition altitude where the altimeter setting is based on the local pressure. This is crucial during descent or arrival, as it prevents any potential altitude discrepancies between different aircraft operating in the same airspace.

At the transition level, the standard altimeter setting switches to the local altimeter setting, which reflects the current atmospheric pressure at sea level for the area you're operating in. This practice ensures that all pilots in the vicinity are using the same reference for altitude, which enhances safety and situational awareness.

In contrast, adjusting the altimeter at points such as the top of descent, the final approach fix, or at the transition altitude does not ensure that all altitude information is based on the local pressure setting, which is vital for safe altitude and vertical separation in a controlled airspace. Thus, the correct procedure emphasizes the significance of transitioning at TLv for accurate altitude readings during descent or arrival.