A hardware boundary is the practical limit of what a device is built to accept. It covers the electrical, protocol, and format constraints that separate supported inputs from unsupported ones. In a monitor, for example, the boundary may include which connector types, signal timings, voltage ranges, and handshakes the hardware can safely process.
In cybersecurity, hardware boundaries matter because attackers often probe them with malformed or unexpected inputs. If a device accepts more than it was designed to handle, that extra flexibility can expose bugs in firmware, parsers, or switching logic. Defenders use clear boundaries to reduce attack surface: they validate inputs, reject out-of-range signals, and document supported modes so users know what is safe. In practice, a strong hardware boundary helps prevent crashes, lockups, and unsafe behavior when devices face unusual or hostile input.