Output Unlock Conditions
The redesigned UTXO model allows for implementing additional logic on transfers. Outputs that are The results of a transfer may define special unlocking logic. When these outputs are consumed in subsequent transfers they have to satisfy all unlock conditions that are defined on them.
Outputs are locked under addresses that can be unlocked via a signature.
Storage Deposit Return
Due to the storage deposit rules, creating an output with less than the minimum required storage deposit funds is impossible. So how does one send such small amounts?
The new storage deposit return unlock condition allows to specify a return amount that has to be refunded to the sender's account. Therefore, if one wants to send 1 token to someone but the minimum deposit is 10, they can send 11 tokens to the recipient and await 10 back. There is no cheating; the recipient has to send 10 tokens back if they wish to own that 1 token.
In the previous example, what if the recipient never consumes the output? The sender's tokens are locked forever, even though they are not in the recipient's possession. A cautious sender would also define an Expiration Unlock Condition on the output. As the name suggests, such outputs expire after a deadline, meaning that the sender regains full control of the tokens in the output.
Expiration is also an important feature for smart contracts, as one might decide to cancel an on-ledger smart contract request if it is not processed within a deadline.
Outputs may be time-locked, meaning no one can unlock them before a specific deadline is passed. With such a feature, it becomes possible to time smart contract requests in the future. An NFT auction organizer, for example, could prepare an on-ledger smart contract request that closes the auction at a given date and time, transferring the to-be-sold NFT to the highest bidder at that moment.