Custom Fields : Allow for WP/plugins

Hi !

I've installed a new version of Wordpress with Multisite sets on. I've only installed CustomPress and Update Notifications.

First of all, what does "Allow for WP/plugins" means under the "Custom fields" tab ? Then, when I create a custom field with "Allow for WP/plugins" checked, everything is ok. But as soon as I edit this custom field later, for example the description of the custom field, "Allow for WP/plugins" is unchecked after saving.

Could you please fix that ?