Inspectionability is how easily a system can be checked, understood, and verified after it has been built. In cyber security, this means being able to review firmware, configuration, logs, wiring, permissions, and other state well enough to confirm that the system is doing what it should. A highly inspectable system leaves clear clues about its behavior; a poorly inspectable one hides faults, mistakes, and tampering.
It matters because attackers often rely on opacity. Hidden services, modified firmware, undocumented dependencies, and unclear control paths can make malicious changes harder to spot and slower to remove. Defenders improve inspectionability with good documentation, asset inventory, secure boot, code signing, configuration baselines, logging, and independent verification. In a custom build, like any device with added electronics, the same principle applies: if the hidden layer cannot be easily examined, trust becomes weaker and maintenance becomes riskier.