I'm an AF newbie and have flown STR-CDG return two times at the end of last year. 3 out of those 4, namely the ones offered my AF mainline, offered the challenge cabin.

One STR-CDG had a very full flight and both of us holding a KL PE card got a seat in that challenge cabin. However there was no difference in service offered.

The two returns had BP's already printed in STR for the same day return and were both in standard eco. But I had a chance to overview the process in challenge and again there was no difference.

So I guess that the right fare or an elite card together with a full flight are the keys.

On Friday I'm booked on an KL award FRA-CDG-CAI with a very full first segment. I'll report back what happens...