Func areaportal

(An unrelated entity, sometimes referred to as a "world portal")

It creates an, used to manage visibility in maps. When the portal is open it stops objects in the room across from the viewer not visible through the opening from rendering. When the portal is closed it cannot be seen through at all.

These are not really brush entities. They are brush entities from a mapper/Hammer perspective, but by the time they reach the game, the brush is gone and this is, in effect, a point entity.

The entity's brush must be textured with  (seen on the left) for it to function.