Hey there,
At this point I don't know, because the details are so vague. I wonder though, if the letter is different for everyone, because their exact words to me regarding the Main List were "Instances when applicants on the Main List decline or are found to be ineligible are rare." Or is this what you are pertaining to by "slim possibility of moving to the Main List"? I'm not banking on getting on the Main List, although I would like to know more about "It also means you will be prioritized for other funding outside of the Erasmus Mundus scheme." There's been mention of Geographical Windows on this thread, and I wonder if MediaAC also has that, and whether one must first (like you said) accept the self-funded/external-funded placement offer to be prioritized for these. But from what I gather, they send out external funding offers together or shortly after the Main/Reserve notice. Well, I guess it's best to ask the Secretariat. I would actually try to ask them about my exact ranking too, I don't know, it might be kind of disheartening to find out I'm #39 out of 40 haha. Keep in touch and I hope it works out for you!