UPU 4-State barcodes are used for adding identifier tags to UPU postal items. They can be either 57 or 75 bars in length.
The UPU 4-state barcode encodes the following data for S18C and S18D barcodes:
| Data Element | Length |
| UPU Identifier | 1 |
| Format Identifier = 18C | 3 |
| Issuer Code | 3 |
| Equipment Identifier | 3 |
| Item Priority | 1 |
| Serial Number- Month | 2 |
| Serial Number – Day | 2 |
| Serial Number – Hour | 2 |
| Serial Number – 10 Min | 1 |
| Serial Number – Item Number Part | 5 |
| Tracking Indicator | 1 |
| Data Element | Length |
| UPU Identifier | 1 |
| Format Identifier = 18D | 3 |
| Issuer Code | 3 |
| Equipment Identifier | 3 |
| Item Priority | 1 |
| Serial Number – Item Number Part | 12 |
| Tracking Indicator | 1 |
This barcode type uses a Reed-Solomon error detection and correction scheme.