An extended public key is the watch-only half of a hierarchical wallet: from one xpub, a merchant derives unlimited receive addresses without ever exposing a key that can spend. The checkout generates a fresh address per invoice; the spending keys never left cold storage.
The pattern buys privacy, no address reuse, no change clustering, and security, the surface that watches cannot move funds. It is how serious Bitcoin acceptance has worked since the early blocks.
Bitcoin checkouts on U.CASH can settle to an xpub: every invoice gets its own derived address, and the merchant's spending keys stay wherever the merchant keeps them.
Every term behind the rails, the assets, and the settlement.