Manipulators are objects you can swap back and forth into a scene graph at locations where you have an SoTransform node.

They take the place of the SoTransform, continue to act like the SoTransform, but in addition give the user a 3D user interface, using draggers, for controlling the transform. When the user is done manipulating, the manipulator can swap itself back out, replacing itself with an SoTransform that represent the new transform value.

Manipulators is one of the concepts that really made Open Inventor stand out against the alternatives when it came out.

See also: Dragger Classes