Yes, I'm assuming thats from the fabricator? What they mean is if you just go for one then it'll be fitted into the casement opener, if you want two then for symetry they'll put both in the frame head section but in order to do that they propose to use a slightly thicker pvc section usually found on door frames, they do this to give room for the vent, if they fitted them into the standard window section of frame theres a 99.9% probability the vents would be buried into the plaster