Thanks Joakim, nice explanation !It is quite similar to GPH, but uses all wind strengths with the distribution given e.g. here: https://orc.org/index.asp?id=33
So both are for the "round the island" (or "circular random") course with even wind direction distribution.
GPH is the average of 8 and 12 knots wind and is thus for rather light wind.
APH uses all the winds from 6 to 20 knots, with an average of 12 knots.
Actually GPH is a bit more complex than that, since it uses wind averaging, which is not used for APH. This makes boats clearly slower in light and medium wind. Maybe it is better not to go into details of wind averaging, since it is no longer used for anything else than GPH, which is shown just for comparison with old certificates.