Forum:Suppression Fields

Could someone clarify the exact functions of the Suppression Fields? Or give me a proper definition? Obviously they prevent humans from reproducing, but they also seem to restrict movement. CP and trains can pass through them, as well as citizens in Point Insertion, yet Freeman can't. I've been debating this with my brother (also a hardcore Halflife fan) - his theory is that the Suppression Field mentioned in Breen's speeches and NPC dialog refers to some single field surrounding the City 17 environs and not the fields seen in and around the city. I've always assumed that 'Suppression Field' referred to any of the fields seen in the game. I tried to look it up on the wiki, but only found Suppression Device, which we all know and love. Can someone help me out here? - Zasmyenok