A license is not technically mandatory, but the nanopublication network requires that published nanopublications be openly shareable and replicable, so any license that is declared should be compatible with that. In practice, most nanopublications point to a permissive license via the dct:license predicate in their publication info — typically CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/) or CC0 (https://creativecommons.org/publicdomain/zero/1.0/). Because the license is part of the signed, immutable nanopublication, it travels with the content and is verifiable by anyone who later retrieves it.