200mm/250mm/300mm Scribing Tool Screw Cutting Marking Gauge Mark Scraper Tool Fo

How is this thing intended to be used? It is totally unclear to me as the dimensions on the device do not line up with any edge or the end. I really don't understand it and I hope you can help with some actual detail. Even the image does not show it lining up with an edge of what it is presumably intending to scribe some line on. Please explain, especially as it has a vernier and seems to be intended for some accurate purpose but the end of it is not an accurate distance from the end????? Thanks very much

Not a question, but a statement: I wish that these Bangood tools would have the option of Imperial scales on them. Or maybe both metric and Imperial... I would buy them if they did.

Not a question, just thanks to Stewart who took the time and answered my original question. I had already bought one of these and found it a solid and useful device, but had failed to switch on to it's measurement function... Grey matter obviously not what I used to be so Thank you Stewart