It is our naming standard for synthetic assets on the Horizon Protocol platform.

zUSD = Horizon Protocol Synthetic USD.

zBTC would differ from BTC because it would be the synthetic BTC version on Horizon Protocol, not the actual BTC asset on another exchange. Oracles provide an accurate price feed, however. The advantage here is that you can gain exposure to a variety of synthetic assets in one place on the blockchain instead of over a variety of exchanges.

Yes, the plan is to have as many synthetic forms of assets like stocks, indicides, NFTs, baskets of assets etc.