Texinfo files start with the line:
\input texinfo
The ‘\input texinfo’ line tells TeX to use the texinfo.tex file, which tells TeX how to translate the Texinfo @-commands into TeX typesetting commands. (Note the use of the backslash, ‘\’; this is correct for TeX.)
It makes sense to include any command that affects document formatting
as a whole in the header. The @settitle line is usually
present at the beginning of the header:
@settitle Sample Manual 1.0
The @settitle line specifies a title for the page headers
(or footers) of the printed manual, and the default title and document
description for the ‘<head>’ in HTML. @synindex
(see @synindex: Combining Indices), for instance, is another command often
included in the header.
The start of the Texinfo file up to the first content that is output
as part of the main body of the document is the preamble.
It includes the header, Document Permissions and Title and Copyright Pages
specification. It is important for the LaTeX output format as the
end of preamble is where the \begin{document} line is output. In
other output formats it may be used to determine how some special output is
formatted, for example @copying: Declare Copying Permissions output as a comment
at the beginning of output files, or the language used in file headers.