PNB are the expired domains with very high authority. they are known as the PNBs. SEO expert buys that kind of domain and link to it for the authority back links this is called the pnbs.
In SEO, PNB refers to Private Network Blog. It is one blog website that contains group of blogs and it is managed by 1 admin for link building purpose. Sometimes it's safe to use sometimes not.
Private Blog Networks, PBNs, are generally groups of blogs or sites often controlled by one publisher, with the goal of building up links within the network as a way to help particular content rank better in Google.
A PBN is a set of domains that you control which all link back to your money-site. Typically PBNs are made of expired domains that have great back links and therefore pass high levels of link equity to your money-site