'nb' is an interwiki alias for the Norwegian Bokmål wikipedia (it's the actual ISO 639-1 code for Bokmål). This alias is currently not recognised by the "add links" interface on Wikipedia.
It would be beneficial if this interface did recognise this alias, as this code (currently at least) does not give any suggestions at all, whereas 'no' gives several. For example, typing in 'no' and hitting tab gives 'frrwiki' as the chosen wiki. If 'nb' were active, it would give the right wiki with simply code + tab.
Furthermore, the code 'nb' is often used in hard-coded interwiki linking on the Norwegian Nynorsk wikipedia, so it would be beneficial for users there if this alias were active, either way.