The reactor Plane object is a type of rigid body that acts as a fixed, infinite plane in the simulation. It shouldn't be confused with the standard 3ds Max plane, which can also be used as a rigid body provided its is set to Concave Mesh.
Like other rigid bodies, the Plane can have display proxies assigned to it and can be contained in a compound rigid body. You can also assign friction and elasticity values to it. However, it is always fixed in the simulation (the and properties are ignored).
The reactor Plane acts in only one direction, as shown in the viewport by an arrow pointing away from its solid surface. This means that rigid bodies approaching the plane from the "wrong" direction pass through it. You can, of course, also use two opposing planes.
The reactor Plane as rendered in a viewport