Both parts are equally important obviously but yeah I guess since the light reflects off the white part. If it's optical like on the phone then it's just doing it optically based on regular visible light. There are actually a ton of different barcodes people have come up with, used in different areas. QR codes are the obvious recent ones that are pretty big. They're more sophisticated because they use error correction to be more reliable. And of course a lot of them have standards that specify how to interpret their data and so on. https://en.wikipedia.org/wiki/Barcode#Types_of_barcodes