This option enables you to specify from what session key the group list is to be polled. This session key should then contain a comma separated list of groups that will have access to the file system.
Example of usage in the config.php file:
$mcFileManagerConfig['authenticator.session.group_key'] = "FileManager_Groups";
Example of usage in a mc_access file:
authenticator.session.group_key=FileManager_Groups