Making your way down the aisle can be an inconvenience so sometimes you will be allocated seats near the front of the aircraft. It is likely that you will be asked to sit on a window seat – which may mean transferring across the adjacent seats. If you are using special seating supports there may be restrictions governing where this can be used. For example the TravelChair can only be used where there is no passenger sat directly behind – frequently with the bulkhead directly behind the occupant.
A bulkhead is a physical partition that divides a plane into different classes or sections. Typically, a bulkhead is a wall but can also be a curtain or screen and can be found throughout the plane, separating the seats from the galley and lavatory areas. There are pro’s and cons of bulkhead seats, for more information see here.
More information on aircraft seat pitch.