The unsafe L2 head: blocks between the safe and unsafe heads are unsafe blocks that have not been derived from L1. These blocks either come from sequencing (in sequencer mode) or from unsafe sync to the sequencer (in validator mode).

Optimism allows asynchronous calls between L1 and L2 users or contracts. Practically, this means that a contract on L1 can make a call to a contract on L2 (and vice versa). This is implemented by deploying “bridge” contracts in both Ethereum and Optimism.