Default Privacy vs Optional Privacy: The Main Difference
Default privacy means that privacy is applied automatically as part of the normal transaction model. This is the Monero approach: XMR transactions are designed to hide sender, receiver and amount details without requiring the user to choose a separate privacy mode.
Optional privacy means the user or wallet can choose between visible and private transaction types. This is the Zcash approach: ZEC can move through transparent transactions or shielded transactions, depending on wallet support, service support and user selection.
For Zcash, privacy depends not only on the protocol, but also on whether the user, wallet and service actually support and use shielded mode. This makes Zcash flexible, while Monero reduces dependence on manual privacy-mode selection.