What does rare NFT mean?

Could you please elaborate on the concept of a "rare NFT"? I'm trying to understand its significance in the cryptocurrency and digital art world. Is it solely based on scarcity, or are there other factors that contribute to its rarity? How does it differ from a common NFT? Also, how does owning a rare NFT impact one's portfolio or collection? I'm particularly interested in knowing if there's a specific criteria or characteristics that define an NFT as rare. Could you explain the economics behind it? Thank you for your time and insight.