_DIR_PLUGINS_AUTO is a character strong corresponding to the file path relative to SPIP’s root directory.
It is possible to define this constant in your
config/mes_options.php file (refer to the article dedicated to The mes_options.php file).
Defining this constant with an empty value
'' (an empty character string) means that the automatic installation feature will be completely deactivated.
// Deactivate upload installations from SPIP's admin screens
By default, this constant is set to:
In other words: the "auto/" sub-directory on the "plugins/" folder.