Portal (the game) rules: portals are 2-dimensional holes through space which must be projected onto continuous physical surfaces; attempting to enter a portal from its backside will break that surface and destroy the portal you’re trying to enter.
“2D common portal” rules: portals are 2-dimensional holes through space which exist independent of physical objects in the world. They act like two “Portal” portals of the same colour placed on both sides of a freely-floating surface, paired to a corresponding pair of portals of the other colour somewhere else. Such portals have two sides which work the same way and the only distinction is the direction you exit from the output portal. If a portal is oriented for convenience, entering it from the unintended side will land you facing away from the intended direction (and potentially in the air about to fall into a pit of lava).
“3D common portal” rules: portals are pockets of overlapping space. They act like 3-dimensional versions of the “2D common portal” and have no “back”.