Where there are still packs to be sold in a drop, the hidden supply will mostly show what's left to find in packs. Where the packs for a drop are sold out, there may still be unopened packs. These might be held for promotions and giveaways or they may even be in a private collection.
The private supply are non-minted collectibles that have been found. A large private supply may indicate that collectors are HODLing this moment. It is not necessary to mint a moment to use it in-game or add it to a dropbook.
Listed and unlisted moments form the available supply. This can increase when private moments are minted. Currently, it is time-consuming and costly in fees to mint moments and may not be worthwhile for moments with a very low floor price, so many collectors will not mint unless they want to sell or transfer moments.