#INSERT_HEAD entered between the HTML <head> and </head> markup tags is used to automatically add in certain JavaScript scripts. Some scripts are already added in by default by SPIP (jQuery, for example), and others are inserted by various plugins. Refer to the sections about the insert_head and jquery_plugins pipelines which expand on adding in such scripts.
To add in additional CSS code, it is better to use the #INSERT_HEAD_CSS tag and the insert_head_css pipeline.
In SPIP’s default templates, this tag is inserted at the end of the template named squelettes-dist/inc-head.html.