Originally Posted by
CarmenOM
I would guess that this refers to the fact that, for “domestic” (in this case meaning within Europe) flights KL first class seats are the same as economy seats but the middle seat is not sold.
KLM has also started selling the ability to block the seat next to you in Y unless/until the seat is needed due to pax load (if they end up assigning someone to the seat they refund what you paid).