Chain Code Delegation.

Lets a co-signer of a shared wallet help approve payments without seeing the wallet's balance or other activity.

In shared wallets that need several keys to spend, co-signers usually swap extended public keys (xpubs), which let each of them track every payment. Here one party keeps back part of that data and gives the co-signer just enough to check and sign each payment. It suits an owner who wants privacy from a custodian.