In the truck-scale case, why does the way the weighing data leaves the PC decide whether the system is still isolated?

Because the weighing data is certainly needed elsewhere (inventory, production and billing); if it is passed on without the PC, for example on a paper form, the isolation holds, but if the Windows PC passes it on, the isolation is broken and the communication must be analysed.

The case: company B produces building and insulation materials from recycled material delivered by truck. Each truck drives onto a scale full and again empty; the weight difference gives the delivered quantity. An old Windows PC records the weighing, operated by a scale operator.

Drawing the system (scale, PC, operator inside the boundary) immediately raises the question the sketch leaves open: where does the data go? Inventory management, production and the accounting system all need it.

• Paper form filled in and handed over: the isolation is preserved.
• Data sent by the PC: the isolation is broken, and the communication relationship needs a precise analysis.