On the 787-800, the arm rest raises up and there is a small compartment in the seat. I always put my macbook air in it for takeoff and landing. Been doing this for three years---until today. FA refused to let me do this, even checked that I hadn't done it. I had to stow it up above---and she pointed out the placard--and it is there!--that is on the seat. You would have to bend over and use a magnifying glass to see it. #amazing