If you're talking about the Airbus narrow body fleet. When AC reduced the J cabins, two extra economy rows were added when a row of J seating was removed, and this had to be added in front of the emergency exit rows. So it is best to stay clear of any economy rows in front of the two exit rows, which for some unknown reason, AC designates as Preferred Seating. This doesn't apply on widebodies or the new RJs though.