When creating a world to be simulated or previewed, reactor needs to access the objects in 3ds Max at a fixed point in time. This parameter defines this point in time (in 3ds Max frames). Initial shapes, positions and velocities are taken from the actual shapes, positions and velocities of the objects in the scene at this particular frame. When creating an animation, keyframes are created starting from this frame to the End Frame time.
The number of frames for every keyframe that reactor creates (time step). For example, a value of 2 will create a keyframe every other frame. Increasing this value forces reactor to take bigger time steps, potentially reducing the accuracy of the simulation; you might need to then update the Substeps/Key setting accordingly (see following).
When on, creates a (unless one is already present) for the position and rotation tracks of those rigid bodies using Position/Rotation/Scale transform controllers. A new subcontroller is added to the List controller to hold the keys created by reactor. This new controller is weighted to 100, while the previous controllers are weighted to 0. In the special case of rigid bodies belonging to a character studio , a new biped layer is created every time a reactor animation is generated.