One-of-one and legendary NFT traits are essentially versions of your NFT that have no repetition or traits that can be found elsewhere. This is a good way to encourage value and collectibility in your NFT collection or they can be used as gifts for collectors who have been with you from the beginning. The danger of having one-of-one and legendary NFT traits in your collection is they can harm the rankings of your collection for buyers and affect pricing; it's best to limit these to 10 per 10,000 collection.