Portals - completely interior

I’ve used portals when rendering indoor scenes to direct the light from outside.

Suppose I have an interior, with perhaps an open side on the mesh (image inside a cube with one face missing), lit from the interior with a sphere with an emission shader. Will a portal covering the open side, and pointing into the inside, prevent light escaping? e.g.

If the sphere had an emission shader, and a portal pointing into the cube covered the open face, would this contain the light from the emission? My own tests say the jury’s out.

OK, ignore me. I slapped a plane to see what would happen. The truth is, looking outside the cube there was no difference. The illumination both inside the cube and cast to the plane were identical.

Surely that’s not right? Or does the orientation of the portal not matter?

From my understanding the only point of portals in Blender is to support the sampling of environment lights. I seem to recall that they do not affect other lights at all.

Surely an emission shader works principally the same as an environment light?