Richard A. Edwards
I have the same question. The only authoritative answer I could find is:
They are open for each player to achieve.
They come into game randomly - that way each game set of objectives will be different.
Unfortunately this answer is still very vague.
Other posts suggest they're shuffled into the terrain decks and come into play if drawn. This seems odd to me.
My thought is to shuffle them then draw a number equal to the number of players at setup. Place them near the game board. At the game end any player(s) an fulfill any objectives.
One complaint has been if they come into the game late there's no time to work toward them. By starting with them in play each player can keep them in mind the whole game.
Just my two cents as a player new to this game.