Difference between “hbox group” and “adjusted hbox group”?

Question

eTeX's \currentgrouptype distinguishes two types of \hbox group types

\hbox{\showthe\currentgrouptype} % 3 "adjusted hbox group"
\noindent
\hbox{\showthe\currentgrouptype} % 2 "hbox group"

Looking at tex.web, I see a couple of places where hbox_group occurs:

@d hbox_group=2 {code for `\.{\\hbox}\grp'}
@d adjusted_hbox_group=3 {code for `\.{\\hbox}\grp' in vertical mode}

@<Initiate the construction of an hbox or vbox, then |return|@>=
...
  if (box_context<box_flag)and(abs(mode)=vmode) then
    scan_spec(adjusted_hbox_group,true)
  else scan_spec(hbox_group,true)

@<Cases of |handle...@>=
hbox_group: package(0);
adjusted_hbox_group: begin adjust_tail:=adjust_head; package(0);
  end;

I don't understand what adjust_tail:=adjust_head does. Perhaps something to do with the \vadjust command?

Answer 1

Whenever TeX starts a new level of grouping it records the current type of group so that it can do the right kind of action if a group ends. If hboxes are build it distinguishes between hboxes in horizontal mode (group code 2) and those in vertical mode (group code 3).

What is different betwen the two is that in the vertical case ins_node, mark_node and adjust_node nodes (from \insert, \mark and \vadjust) migrate out of the box into the outer part.

They are collected in the hpack routine in the list starting at the pointer adjust_head. Then when the group ends (group code 3) the head of this list is assigned to the tail of the list ending at adjust_tail and this is what you see in the above code happening.

See also §647.