From ResourceSpace permissions page:
U (upper case) Can manage users in children groups to the user's group only
From ResourceSpace config.default.php:
"U" permission allows management of users in the current group as well as children groups. TO test stricter adherence to the idea of "children only", set this to true.
$U_perm_strict=true;
From ResourceSpace permissions page:
From ResourceSpace config.default.php: