Yes, this should be assumed to be known. The tool location in the machine frame is found using limit switches on the machine. There are three limit switches, for X,Y, and Z.
I haven’t though about this before, but yes that should be possible! You could theoretically use the sensors to align with an edge or corner of the workpiece. They are able to detect lift-off (or when they stop getting readings), so they would be able to tell when they move off the workpiece (or when they move back on). There could be a shnazzy little interface to show the sensors’ status so that you could line it up as close to the edge as possible. To go even further, you could theoretically get the camera feed from the sensors so that you can actually see where on the edge you are. That would require a lot of processing power, though.
But yes, I agree that being able to reference edges is a pretty huge feature.
Yuppp, as @Tokoloshe mentioned, I have been using a B&D workmate for this. It’s not perfect (it’s got a bit of slop that sometimes makes the top planes bow if clamped too hard), but it does the trick.