Use of \@texttop and \@textbottom for vertical positioning

Question

There are two macros defined in the LaTeX kernel named \@texttop and its opposite \@textbottom. The first is a bit of a mystery command in that in the source it is let to \relax. There is a note that the:

\@texttop : Command executed at top of vbox holding text of page (including figures). Used by letter style; can also be used to produce centered pages.

In the letter class it is set to a small value:

The document class letter sets |\@texttop| to |\vskip| 0pt \texttt{plus} .00006fil 
%    on the first page of a letter, which
%    centers a short letter on the page. This fil value may have to be
%    changed for other letterheads.  This setting has to be done after
%    |\raggedbottom| is executed, since the latter sets |\@texttop| to
%    |\relax|.

From my understanding in reading source2e, these are ideal hooks to insert top and bottom glue in the final output box. For example in a book of poems or a photo-book, we might want all the pages to be vertically centered. I have defined a short macro, shown in the minimal below named \nobottom to supplement the commands \raggedbottom and \flushbottom.

\documentclass{article}
\usepackage{lipsum}
\usepackage{graphicx,alltt}
\makeatletter
\def\nobottom{%
\def\@texttop{\ifnum\c@page>0\vskip \z@ plus 3fil\relax\fi}
\def\@textbottom{\ifnum\c@page>0\vskip \z@ plus 2fil\relax\fi}}
\nobottom
\begin{document}
\pagestyle{headings}
\lipsum[1]
\clearpage
\lipsum[1-2]
\clearpage
\begin{alltt}
This can be a poem
     to see how it will
print
\end{alltt}
\end{document}

Is my understanding of these commands correct? Is there another way to vertically position all pages or certain pages automatically? As you can see at the point of time LaTeX inserts these commands, it is safe to use the page counter, so for example one could automate the centering of all pages containing some preamble material (or these pages could be marked with a boolean to affect such an insertion of glue). Can you provide some other possibilities and examples for such commands?

Example for images (I think achieved in a much easier way than recent posts we had for the same subject i.e, caption on left and image on right).

Answer 1

Your reading is correct. Those two commands are already part of the initial LaTeX 2.09 output routine (the documentation in ltoutput.dtx clearly show the age --- if something in typewriter looks like this then this is Leslie original):

%   \@textbottom    : Command executed at bottom of vbox holding text of
%                     page (including figures).  The \raggedbottom
%                     command almost \let's this to \vfil (actually sets
%                     it to \vskip \z@ plus.0001fil).
%                     Should have depth 0pt.
%
%   \@texttop       : Command executed at top of vbox holding text of
%                     page (including figures).  Used by letter style;
%                     can also be used to produce centered pages.
%                     Let to \relax by \raggedbottom and \flushbottom.
%

And indeed they offer the only way that I can think of (short of redefining internals) to automatically center page material (or position it 2/3 as you did).

The name \nobottom is a bit odd though, isn't it?