Well, in theory the first X number of seat rows behind the curtain should be reserved for Elites and full-fare (X, S & B, I believe) economy pax. This is called the Select Seating zone, and the idea is that middle seats should be kept free if at all possible. Seats in this zone should not be available to other pax during OLCI.

In my experience, Select Seating procedures are rarely adhered to. Even on flights that are far from full all sorts of pax are allocated seats in that zone, and middle seats are more likely to be occupied than not.

Johan