boost::multi_index::ordered_unique< boost::multi_index::composite_key< element_t, boost::multi_index::member< element_t, unsigned char, &element_t::x >, boost::multi_index::member< element_t, unsigned char, &element_t::value > > >
As it is not clear in the documentation what the "boost::multi_index::ordered_unique" scope is.
The scope is the key you define under "ordered_unique". In the above example you define a unique key that consists of (x, value). I.e., the container would disallow duplicate (x, value) pair.