Throughout the contracts, with every mint there is often an associated index which is an u64. The reason for this is that while you may have a single NFT, you may wish to use it across multiple games. These different games may not wish to share a Player stat sheet or an Item definition. This is why all NFTs can have multiple Items and Players defined on top of them using the index as part of the seed. This index allows a single NFT can be used in different ways by different games.
Another common use-case might be to define index 0 of an NFT as an ItemClass, and index 1 as the Item for a true 1/1 Item so that you don’t need to have two NFTs to define a single legendary sword. Same could be done with a PlayerClass/Player.
It should be noted that if one Item PDA describes a Consumable that destroys the NFT, and you use the NFT in this way, it will be destroyed, regardless of its other definitions.