Machines list filters - Exposure Level

I've spotted what looks like a relatively new filter in the machines list, Exposure Level and I'm trying to find out what this filter is based on as compared with the Risk Level filter. Risk level is well documented: "The risk level reflects the overall risk assessment of the machine based on combination of factors, including the types and severity of active alerts ... etc."

If I click on the Exposure Level "high" filter it produces a very odd list of machines in our environment - I cannot fathom what it is trying to tell me ! Anyone able to point me in the right direction please ?