_DIR_PLUGINS_SUPPL is defined as follows:
Example:
phpdefine('_DIR_PLUGINS_SUPPL', _DIR_RACINE.'squelettes/' . ':' . _DIR_RACINE.'sites/'.$site.'plugins/');
We assign a value that includes the pathname of any supplementary plugins defined as from the site root directory (of the master site if it entails "mutualisation" or core kernel sharing),.
Using this constant, it becomes a much simpler matter to manage several plugin folders for a site (very useful for multiple SPIP sites residing on a single SPIP code installation, for instance).
For example:
define('_DIR_PLUGINS_SUPPL', _DIR_RACINE . 'sites/' . $site . '/plugins/');