# Where is the reference point of a VBOX?

From the TeXbook,

Therefore, the actual rules for the depth of a constructed vbox a somewhat TeXnical. Here they are: Given a vertical list that is being wrapped up via \vbox, the problem is to determine its natural depth.

1. if the vertical list contains no boxes, the depth is zero.
2. if there's at least one box, but if the final box is followed by kerning or glue, possible with intervening penalties or other things, the depth is zero.
3. ...
4. ...

my question is about item 2, in the following code snippet,

% Exercise 12.12
% \box1 is 1pt high, 1pt deep, and 1pt wide;
% \box2 is 2pt high, 2pt deep, and 2pt wide;
% \baselineskip = 9pt minus 3fil
\setbox4=\vbox to4pt{\vss\box1\moveleft4pt\box2\vss}


as the book states, the natural depth for the vertical list (in this case, it is 0pt plus 1fil minus 1fil, \box1, 6pt minus 3fil, \moveleft4pt\box2, 0pt plus 1fil minus 1fil) is zero. When the vertical list is preparing to be wrapped up into a vbox (hasn't been set glues), is the reference point of this vbox the position 4pt right 2pt down from the reference point of box2 (at the bottom line of box2)? After setting the glues according to the height of this vbox and the natural height of this vertical list, the reference point of this vbox goes up a bit?

If the last item of a vertical list is a glue, it makes me think that if there is a vbox just fitting this vertical list, the reference point of this vbox is located at the bottom line of this vbox.