NFTs can be minted with arbitrary attributes. And those attributes can then be used to generate rarity charts and other types of indexes and interfaces.

Some common attributes I've seen on NFT platforms like OpenSea and RareCandy:

• color - normal set, unique
• background - unique
• extra object (hat, glasses, scarf, shoes)
• object orientation
• generation (eg: first, second, third)

Ultimately, rarity.