A start-of-header line is a Texinfo comment that looks like this:
@c %**start of header
Write the start-of-header line on the second line of a Texinfo file.
Follow the start-of-header line with @setfilename and
@settitle lines and, optionally, with other commands that
globally affect the document formatting, such as @synindex or
@footnotestyle; and then by an end-of-header line (see End of Header).
The start- and end-of-header lines allow you to format only part of a Texinfo file for Info or printing. See texinfo-format commands.
The odd string of characters, %**, is to ensure that no other
comment is accidentally taken for a start-of-header line. You can
change it if you wish by setting the tex-start-of-header and/or
tex-end-of-header Emacs variables. See Texinfo Mode Printing.