HITL validation focus across representative use cases

HITL provides an expert, safety-focused audit of LLM-generated PLC code in native engineering environments (Siemens TIA Portal, Beckhoff TwinCAT). It serves both as the acceptance gate for deployment and the reference signal used to calibrate automated validators. For each use case, the review verifies four aspects: compilation for syntactic correctness and build integrity; simulation/runtime behaviour covering sequence logic, timers, and interlocks; safety and regulatory compliance—including fail-safe modes, E-stop dominance, and watchdogs; and maintainability assessed from structure, naming, comments, and modularity. The representative outcomes for simple, medium, and complex scenarios are summarised in the attached table; the complete matrix spanning all 25 industrial cases is available in the supplementary material (and Zenodo archive). This protocol yields auditable ground truth, captures vendor-specific nuances that automated metrics may miss, and provides high-precision labels for calibrating LITL thresholds and prioritising risk-critical fixes.