They are given a notional seat just before OLCI. So you won't see it on the seating plan several days before OLCI; but at OLCI these passengers will generally open up the screen to find what the system has offered them, and can at that point change it. If the cabin is over-booked a more complex logic applies, typically with some passengers only allocated a seat at the airport. Consequently if you are a status passenger, and you see lots of seats free say 4 days before flying, you have no clue how full the service will be.