A serial number is a production identifier — unlike date codes (which encode production location and timing but aren't unique per bag), a serial number is a unique string tied to a specific unit. Chanel uses serial numbers on hologram stickers and authenticity cards. Gucci embosses serial numbers inside bags on small leather tabs. Hermès uses blind stamps for year of production and craftsman identification.
Serial number formats change over time as brands update their systems. Cross-referencing the format and length of a serial number against documented authentic examples from the claimed production year is an essential authentication step.