A common type of thread in the Discussion forums. AH Challenges have many varieties.
The best kind amounts to “How can we get this situation by (year), plausibly, suggest all your ideas and then I will give you mine and write a timeline.”
The worst and rather more common kind amounts to “Justify my political prejudices and when you fail to I shall get angry and incoherent”.
Having said that, several good timelines have started from AH Challenges, not always written by the person who posed the challenge.
A subset of AH Challenge are the Map and Flag Challenges, in which a member posts a map or a flag and everyone else has to suggest how history has changed to get that map by the given year. This is often good fun and has also sparked timelines. The original poster may or may not have ideas of their own about how we got to that situation, and what is often most interesting is to contrast their ideas with what other people say. However, it is often also lazily used by certain people who scrawl implausible maps with no idea of how to get there, sparking Gladi's famous quote.