Meno – there’s a section of the plane that is adult-only. The airline people do not give any of those seats to children below a certain age. Any adult who requests a seat in that section is given a seat in that section. I assume they usually have to pay extra, but maybe not.
All adults who don’t specifically request a seat in that section are not given a seat in that section (unless there are extra seats in that section and not enough seats in the regular section and they are not travelling with children).
That is how I understand it, but I guess those who know more about it can correct me if I’m wrong.