Q:

What is the semantic meaning of 'this.object.position' in Three.JS's trackball?

I've spent some time trying to figure out what this.object.position is supposed to represent in this Trackball control: https://github.com/mrdoob/three.js/blob/master/examples/js/controls/TrackballControls.js
The author not only saves the original value of position (for reset purposes) but also keeps track of the lastPosition whenever the position changes (or not) after each frame.
... // the first half of the constructor

var lastPosition = new THREE.Vector3();

... // more initialization

// for reset
this.position0 = this.object.position.clone();

At first I believed that it stood for the position of the camera, but there's also a 'this.eye' variable, so that would be a conflict. Then I thought that it had something to do with the position of the drawing surface on the DOM, but since it's a size 3 vector that theory doesn't hold much weight either.
I know this may be a stupid question, but I will really appreciate a nudge in the right direction.
Thanks for reading

A:

this.object.position is the camera position. (see note)
this.eye is the vector from this.target to this.object.position. The target is the point  the camera rotates around.
_eye.subVectors( _this.object.position, _this.target );

Note: This is assuming that the camera is not a child of another translated or rotated object -- in which case, the control will not work correctly.
three.js r.58

