Each plugin may set its own settings on each Item/Post. These settings will be stored in the database and can be requested any time, when the Item object is available.

How to set a custom setting:

$Item->set_setting( 'setting_name', $setting_value );
$Item->dbupdate();

Note that no changes are written to the database until you call $Item->dbupdate();. This is so that you can queue several changes before updating the DB in one single operation.

How to get a custom setting:

$Item->get_setting( 'setting_name' );