niconico video has some prefix for video ID, like:

• sm: uploaded by normal user (name comes from SMILEVIDEO)
• so: uploaded by official channel (SMILEVIDEO OFFICIAL?)
  • like https://www.nicovideo.jp/watch/so26654381
• nm: uploaded by normal user, but upload from software "niconico movie maker"
  • like https://www.nicovideo.jp/watch/nm6049209
• ... and more (now deprecated) prefixes... see https://dic.nicovideo.jp/a/id

but in current MusicBrainz, only sm prefix are allowed.