What determines spanwise placement of spoilers for roll control?
On airliners, spoilers serve three purposes, roll enhancement (assist the ailerons when greater roll rate is commanded), speed brakes (help slow down and descend faster), and ground spoilers (kill lift in order to transfer weight from the wings to the wheels for more effective braking). Spoiler span-wise location is a compromise to accomplish all three goals. Also, the outboard spoilers are primary for roll control and the most inboard spoilers may only be used on the ground. Spoilers are not placed in front of the ailerons because they would interfere with roll control. The chord-wise location leaves the wing box clear for fuel tanks.