Apple Serial Number is a unique alphanumeric identifier that Apple assigns to every device. It contains information about the model, date and place of manufacture.
Since 2010, Apple uses a 12-character format. Older devices may have an 11-character number. Only uppercase Latin letters and digits, no letter 'O' (zero is used instead).
Difference from IMEI: serial number is a device identifier (model, warranty, GSX). IMEI is a mobile module identifier (Blacklist, Carrier Lock). Learn more: IMEI vs Serial Number.