The registry table is an hidden table shared between vm and all his thread(friend vms). This table is accessible only through the C API and is ment to be an utility structure for native C library implementation. For instance the sqstdlib(squirrel standard library)uses it to store configuration and shared objects delegates. The registry is accessible through the API call sq_pushregistrytable.
void sq_pushregistrytable(HSQUIRRELVM v);