It is a unique ID number or code assigned to a package or parcel.
When shown on a receipt after shipping a package the tracking number is usually close to the barcode.
DHL Express supports the carrier-independent ISO standard 15459-1[6] for the identification of single packages.
Such identifiers consist of uppercase characters and numerals, and they can be up to 35 digits long.
Companies that already use this standard do not require an additional identifier to track their pieces when shipping with DHL Express.