The constant _HEADER_COMPOSED_BY defines the content of the Composed-By part of the HTTP header of pages generated by SPIP (see article $GLOBALS[’spip_header_silencieux’] on this subject). By default, this constant is set to "Composed-By: SPIP".
It is possible to customize this value in your file
config/mes_options.php (See the article dedicated to it).
Example :
// do not send a Composed-By HTTP header
define('_HEADER_COMPOSED_BY', '');