There is a drop down menu that allows you to select a PG material type whether or not you have the QR. It’s good if the machine doesn’t properly read the code or the label is missing or even if the material is flipped and the code is on the bottom.
The UI allows you to choose settings for a proofgrade material if no QR code is found. For example I chose medium Cherry plywood settings for this piece which is actually just some 1/8" luan I had laying around
It appears to me (and I believe @dan has stated) that each piece has a unique code. I’m not sure what Glowforge’s plans are for the future, but in theory they could “disable” individual codes as they believe the material has been used up. I imagine this is done to prevent third parties from making counterfeit Proofgrade. Also as @rpegg has mentioned, as long as we’re not locked out from using the Proofgrade settings if the material isn’t detected, it’s no big deal.