What’s the difference between public and private category?

Public category is for creating NFTs or SBTs containing files that are visible publicly on the blockchain. Private category only stores the file hash string (such as 4669105d5a7440f09650d63986ccc1161efab6a252a1e5e5ddbb425410252564) in the NFT.

Typically, you’d always use the public category. You would only use the private category when you need to notarize a personal file that you do not which the public to access. Thus, the file hash acts as proof of your file’s existence by carrying its hash string on the blockchain.