This ERC proposes a mailbox API and message format for sending and receiving data between L2s. It is intentionally designed to be general, providing a foundational standard for building application-specific messaging protocols. This ERC is analogous to the Internet Protocol; it focuses on a format for sending and receiving messages, but leaves everything else up to higher-level protocols.
This ERC has similar motivations as ERC-7786. Both standards offer general cross-chain interfaces, but differ in how messages are represented and stored on chain. It may make sense to combine this ERC with ERC-7786 or vice versa.