For example with a search of the following expression: "SPIP is a publishing system for the Internet".
The search will be based on the one hand on the complete expression "SPIP is a publishing system for the Internet", on the other hand on the words "SPIP", "publishing", "system", "Internet".
It is possible to customize the value of the constant _RECHERCHE_MIN_CAR in your file config/mes_options.php (see the article dedicated to it).
Example :
// taking into account two-letter words
// during a multi-word search
define('_RECHERCHE_MIN_CAR', 2);