They are for almost all intents and purposes synonymous, since 90% of Norwegians write in the Bokmål literary standard, which is a combination of Old Danish and the greater Oslo dialects.
There is a subset of Norwegians, around 10%, and especially on the south-west coast around Bergen, that instead uses the Nynorsk literary standard, which is based on rural farmer dialects. There are therefore some purposes (mostly internal to Norway, though also in the cases of huge programs like VLC, WordPress and Windows) where Bokmål and Nynorsk versions should be differentiated.
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