our request has gone before the inspection app looks

PLC sets it and a timer clears it, app doesn't acknowledge receipt at all, which looked fine in the demo because we were watching continuously

Ours looked fine until the display restarted. Then nobody knew whether the request was old or new. Put request identity into the discussion as well as the bits.

I'd have the PLC and application owners agree a retained request and an acknowledgement tied to that request. Include who clears each state, timeout behaviour and what survives restart. Don't just add a sticky bit and stop there.

Does acknowledgement mean the inspection finished, or only that the app accepted the request?

In that proposal, accepted, not finished. Completion needs its own meaning and association with the request. The names on your interface should make that distinction obvious to whoever supports it.

We called both things done on different screens. A very small vocabulary created quite a long meeting.

Right, receipt and result separately, thanks; the controls lead is drawing the sequence with restart cases before we change the interface

Ask what happens when the app accepted the request but the PLC never saw that acknowledgement. That is the awkward case a clean sequence diagram tends to leave politely outside.