The UPU 4-State barcode contains five elements:
- A Leading Quiet Zone
- One Start Character
- All Symbol Characters
- One Stop Character
- A Trailing Quiet Zone
Unlike other symbologies which use bar and space width to encode data, UPU 4-State codes use bar positioning and height for this purpose. Bars can occur in one of four positions or “states.” In the middle third of the barcode’s height, in the top two-thirds, in the bottom two-thirds, or spanning the entire height of the code. This allows postal agencies to encode significant amounts of data without the need to vary bar or space width, which could potentially lead to misread barcodes as parcels and packages are handled during the shipping process.
There are two types of UPU 4-State barcodes: S18C and S18D. Both include UPU identifiers, format identifiers, issuer codes, equipment identifiers, item priority data, serial number information, and tracking indicators. S18C, however, expands the format to include information about the month, day, hour, and 10 minute window during which the serial number was produced.