BNF stands for Backlink not found. It means that when Monitor Backlinks crawled the website it couldn't find a backlink. 

If you imported a link, but Monitor Backlinks was not able to find the link, then it could also show BNF. 

Monitor Backlinks might not be able to find the link for many reasons:
 - the backlink doesn't exist anymore on the site.
 - the backlink changed its location on the site
 - you imported the link with a typo
 - you imported a link generated via javascript (eg. most links on social media sites)

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