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.
The Motor helper lets you apply a turning force to any non-fixed rigid body in the scene. You can specify the angular velocity for the target and the maximum angular impulse the Motor can apply to achieve this velocity.
The Fracture helper object simulates the breaking of a rigid body into a number of smaller pieces as the result of an impact. To do this, you need to supply the pieces that are glued together to create the whole object. This is known as "pre-fracturing." reactor does not break the pieces you supply into smaller pieces.