Petra, AGU is probably not so deliberate in how they place speakers. For special sessions, it is the convener, who is just a scientist like me, who decides who to invite and how to organize the talks-there are a few "invited" talks and the rest are contributed. The rest of the sessions are put together from contributed talks. There is a space when you submit an abstract to say which session or what type of session you want to be in, and whether you prefer poster or oral....Chris