Generally, five quality grades of "A"-"F" are used in the American standard test method. "A" is the best, "D" is the worst, and "F" is the unqualified. Class A barcodes can be read well and are suitable for occasions that scan along one line and only once. Class B barcodes do not perform as well as class A in reading, and are suitable for occasions that only scan along one line but allow repeated scanning. Class C barcodes may require more repetitive scanning. Usually, equipment that can repeatedly scan and have multiple scanning lines is required to obtain better reading results. Class D barcodes may not be readable by some devices. To obtain good reading results, use a device that can repeatedly scan and has multiple scanning lines. Class F bar codes are unqualified and cannot be used.