Intake air bypass, the EX manifold has two stages, one that operates with Oval/square ports and then at higher RPMS, it opens up a second set of shorter runners with circular ports for better high RPM performance - using the first stage for better torque output.

The ECU wether you hookup the IAB or not, will not trip any codes whatsoever. I do recomend getting the EX ECU for the better fuel and ignition timing map, but, if you use the LX it'll still work. Either way, at least hookup the IAB solenoid (you'll notice it being the only real difference between the two) to an empty vacuum port so it'll be a little better to drive around town.