A cashier passes a packet over a scanner. A beep sounds, and the price appears. It is easy to imagine the black lines containing the amount you must pay. For many ordinary packaged products, they do not.
The barcode usually carries an identifier. The checkout system uses that identifier to find a product record, including the price the retailer has assigned.
The stripes are a machine-readable number
In familiar EAN and UPC barcodes, patterns of light and dark widths encode digits. A scanner reads the pattern and decodes the number. The printed digits underneath provide a human-readable version.
Within the GS1 system, a Global Trade Item Number identifies a trade item. Different product variants or pack sizes generally need appropriate distinct identifiers so they can be handled correctly through supply and retail systems.
Think of the identifier as a library catalogue number. It points to a record; it is not the entire record itself. Scanning makes the lookup fast and reduces the need for a cashier to type every item manually.
The retailer decides what the lookup returns
The shop’s product database associates the identifier with information such as a description, current selling price and inventory settings. When the barcode is scanned, the checkout retrieves the relevant details.
This explains why the same packaged product can sell for different prices in different shops while carrying the same barcode. The identifier stays the same; each retailer’s pricing record can differ.
It also explains how a promotion can start without printing a new barcode on every packet. The retailer changes its system, and subsequent scans use the updated pricing rules.
What happens when the scanner cannot help?
A damaged or poorly printed symbol may fail to scan, in which case the printed number can sometimes be entered manually. A readable code can also produce an “item not found” response if the retailer has not set up the product record correctly.
Those are different failures: one concerns reading the identifier, the other concerns what the system knows about it. A barcode is useful only when the surrounding data and processes are working.
The checkout may also apply discounts, taxes or quantity rules after identifying the item. The final amount on the receipt can therefore depend on more than a single stored number.
Some labels carry additional information
Not every retail barcode follows the simple fixed-product example. Variable-measure labels, such as some labels for weighed goods, can encode price or measurement information according to the system being used.
Newer two-dimensional codes can hold or connect to more data, including information useful for traceability. The exact content depends on the barcode standard and its intended use.
The ordinary checkout beep is thus the sound of two systems cooperating: a physical label identifies what is on the counter, and a digital record explains how the shop should treat it. The stripes open the file; the retailer supplies the price.
Sources
GS1 US: Barcode identification FAQs
GS1: General Specifications, including variable-measure identifiers