Private key or another form of it mnemonic seed of your wallet.
Bitcoin is a digital asset so if anyone, any company that produces what can be called as Physical bitcoin, you see high risk here.
They can print a poster, mint a physical item, and attach Bitcoin private key to that item, then sell it to buyers as Physical Bitcoin Collectibles. But they are the entities that get Private Keys at beginning.
You must trust that person, that company that they don't redeem your bitcoins later with private key they have.
Nightmare with yogg.
My Cold Keys Just Got Swiped! All Cold Kuntz!! It can happen with other people, other companies.