Once these relative widths conform to the rules of the quiet zone and guard bars, the reader decides it has found a possible code and begins to look for characters. UPC-A may have been the first ...

“UPC bar codes offer another way for our users ... ScanLife is still the only bar code scanner being preloaded on new phones in the U.S., so it claims to have the widest audience.

Bucklin, Randolph E., and Sunil Gupta. "Commercial Use of UPC Scanner Data: Industry and Academic Perspectives." Marketing Science 18, no. 3 (1999): 247–273.